CN112929094B - High-speed safe optical communication system for key ciphertext simultaneous transmission - Google Patents

Abstract

The invention discloses a high-speed secure optical communication system with simultaneous interpretation of key ciphertext, belonging to the technical field of secure communication. The invention hides the ASE noise with hidden key information in the ciphertext information light again, so that it is difficult for the eavesdropper to find effective information, and the security of the system is increased. By adjusting the optical power, the thickness of the signal is modulated in the optical domain, and the quantum noise stream encryption with higher mask level is realized by using it, and the security performance is improved. The transmission key information is concealed by the white light interference structure, and the key information is hidden in the ASE noise, and the ciphertext information is modulated into the transmission ciphertext by using the thickness of the optical domain. Then, the channel for transmitting the key and the channel for transmitting the ciphertext are coupled to the same optical fiber through the optical multiplexing structure, realizing the simultaneous interpretation of the key and ciphertext, that is, the transmission of the key and ciphertext in the same optical fiber, the device structure is simple, extremely It greatly saves the cost of sharing the key between the sender and the receiver.

Description

A high-speed secure optical communication system with simultaneous interpretation of key and ciphertext

technical field

The invention belongs to the technical field of secure communication, and more particularly, relates to a high-speed secure optical communication system with simultaneous interpretation of key and ciphertext.

Background technique

With the explosive growth of applications related to optical communication, security issues in optical communication become very important. The traditional secure communication technology based on the data link layer will be limited by the bottleneck of electronic processing, and it is difficult to resist the attack of supercomputer and quantum computer, and the security of traditional secure communication technology is challenged. The physical layer-based secure optical fiber communication technology has become a research hotspot due to the advantages of hardware encryption and not being restricted by electronic processing bottlenecks. Due to the physical properties of quantum, quantum key distribution can provide absolute security in information theory, but the device structure is relatively complex, and the key distribution rate is relatively low, making it more limited. At the same time, the current security communication system structure generally adopts the key and ciphertext separation transmission, which greatly increases the cost of the sender and the receiver. It can be seen that improving the key distribution rate, concealing the transmission of effective information, and realizing a system without key distribution is of great significance for improving the security of the secure communication system and reducing the system cost.

In terms of encryption method, the traditional stream cipher is mapped from the codebook generated by the seed key, and the ciphertext and plaintext have the same modulation format. With the improvement of computing power and the development of quantum computers, its security is gradually weakened . The quantum noise stream encryption method uses the inevitable quantum noise in the system to mask the signal. As the number of levels increases, the difference between the levels decreases. The more levels covered by the quantum noise, the security is also improved accordingly. . Due to the limitation of the number of DAC bits and the signal transmission rate, the number of levels that can be covered by ordinary quantum noise stream encryption will be limited. At present, some research groups have achieved high-bit quantum noise in the electrical domain through the thickness modulation of low-bit DAC. Stream encryption, but there are problems such as low modulation depth and signal-to-noise ratio. Implementing an enhanced quantum noise stream encryption in the optical domain is also an urgent problem to be solved.

SUMMARY OF THE INVENTION

In view of the defects and improvement requirements of the prior art, the present invention provides a high-speed secure optical communication system with simultaneous interpretation of key and ciphertext. It realizes enhanced quantum noise flow encryption in the domain; hides and transmits modulation information at high speed through white light interference structure; realizes simultaneous transmission of key ciphertext in one optical fiber through optical multiplexing device.

In order to achieve the above object, according to an aspect of the present invention, a high-speed secure optical communication system for simultaneous interpretation of key and ciphertext is provided, the system comprising: a transmitter, a signal transmission module and a receiver;

The transmitting end includes: a concealed signal transmitting module and a public signal transmitting module; the receiving end includes a concealed signal receiving module, a public signal receiving module and a digital signal processing module;

The hidden signal transmitting module is used to modulate the electric signal of the key information encrypted by the quantum noise stream on the broadband noise optical carrier, and transmit the modulated optical signal as a hidden signal to the signal transmission module. The spectral width is higher than 10nm;

The public signal transmission module is used to modulate the ciphertext information electrical signal encrypted by the quantum noise stream onto a narrow linewidth optical carrier, and transmit the modulated optical signal as a public signal to the signal transmission module. Linewidth is less than 100MHz;

The signal transmission module is used to transmit the concealed signal and the public signal to the concealed signal receiving module and the public signal receiving module of the receiving end respectively;

The hidden signal receiving module is used to receive the hidden signal, perform photoelectric conversion, and recover the key information used for quantum noise stream encryption;

The public signal receiving module is used to receive the public signal, perform photoelectric conversion, and recover the ciphertext information;

The digital signal processing module is used to perform digital signal processing on the recovered key information in combination with the recovered ciphertext information, and decrypt to obtain the plaintext.

Preferably, the public signal transmitting module includes:

The narrow linewidth light source is used to generate the narrow linewidth laser, which is respectively transmitted to the first intensity modulation unit and the second intensity modulation unit after splitting;

a first intensity modulation unit, configured to perform intensity modulation of the first M bits of the PAM electrical signal on one optical carrier, and transmit the coarsely modulated optical signal to the power adjustment unit;

The second intensity modulation unit is used to perform intensity modulation of N bits after the PAM electrical signal on another optical carrier, and transmit the finely modulated optical signal to the power adjustment unit;

The power adjustment unit is used for adjusting the power ratio of the two optical signals and coupling the two optical paths, so that the adjusted coupled optical signal is a uniform M+N bit PAM optical signal.

Preferably, the common signal transmitting module specifically includes the following components:

a laser for transmitting an optical carrier to a third optical coupler;

The third optical coupler is used to split the optical carrier, and transmit all the way to the second polarization controller and the other way to the third polarization controller;

a second polarization controller, configured to adjust the polarization state of the optical carrier so as to be consistent with the principal axis of crystal polarization in the second modulator, and transmit the adjusted optical carrier to the second modulator;

a third polarization controller, configured to adjust the polarization state of the optical carrier so as to be consistent with the principal axis of crystal polarization in the third modulator, and transmit the adjusted optical carrier to the third modulator;

The second intensity modulator is used to modulate the coarsely modulated electrical signal data onto the optical carrier, and transmit the modulated optical signal to the 90° phase shifter;

a third intensity modulator, used for modulating the finely modulated electrical signal data onto the optical carrier, and transmitting the modulated optical signal to the first adjustable optical attenuator;

A 90° phase shifter, used to perform a fixed π/2 phase shift on the received coarsely modulated optical signal, and transmit the optical signal to the fourth optical coupler;

The first adjustable optical attenuator is used to attenuate the finely modulated optical signal and transmit it to the fourth optical coupler;

The fourth optical coupler is used for combining the optical signals and transmitting the optical signals to the multiplexer.

Preferably, the second intensity modulator and the third intensity modulator are Mach-Zehnder modulators for modulating the electrical signal to the optical signal amplitude, and the Mach-Zehnder modulator works in the linear region of the modulation characteristic curve.

Preferably, the signal transmission module multiplexes the concealed signal and the public signal into the same optical fiber for transmission, and demultiplexes the two signals at the receiving end to the concealed signal receiving module and the public signal receiving module.

Preferably, the signal transmission module specifically includes the following components:

The multiplexer is used to multiplex the hidden signal and the public signal, and transmit the multiplexed optical signal to the standard single-mode fiber;

Standard single-mode fiber is used as a transmission channel for optical signals and transmits optical signals to dispersion compensation fibers;

The dispersion compensation fiber is used to compensate the dispersion caused by the optical signal transmission in the optical fiber, and the compensated optical signal is transmitted to the optical amplifier;

The optical amplifier is used to compensate the optical power loss during the transmission of the optical signal in the optical fiber, and the compensated optical signal is transmitted to the demultiplexing filter;

The demultiplexing filter is used to demultiplex the concealed optical signal and the public optical signal, and transmit them to the concealed signal receiving module and the public signal receiving module.

Preferably, the signal transmission module realizes multiplexing through a wavelength division multiplexing device or an optical coupler.

Preferably, the concealed signal transmitting module includes:

A noise light source generating unit, used for generating a wide-spectrum noise optical carrier of linear polarization state, and transmitting it to the first Mach-Zehnder interferometer structural unit;

The first Mach-Zehnder interferometer structural unit is used to modulate the key information of quantum noise stream encryption on the optical carrier phase.

Preferably, the concealed signal receiving module includes:

The second Mach-Zehnder interferometer structural unit is used to match the delay difference between the two channels in the hidden signal transmitting module to within the coherence length of the broad-spectrum noise light source, and then transmit it to the first photoelectric conversion unit;

The first photoelectric conversion unit is used for converting the optical signal into an electrical signal, recovering the key information used for encryption of the quantum noise stream of the concealed transmission, and transmitting it to the digital signal processing module.

Preferably, the public signal receiving module includes:

an optical filter attenuation unit, used for filtering out-of-band noise from the demultiplexed public signal, and attenuating the optical power to a size acceptable to the second photoelectric conversion unit, and then transmitting it to the second photoelectric conversion unit;

The second photoelectric conversion unit is used for receiving the filtered and attenuated public signal, demodulates the recovered ciphertext, and transmits it to the digital signal processing module.

In general, through the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

(1) The present invention hides the ASE noise (Amplifier Spontaneous emission Noise) with hidden key information in the ciphertext information light again, making it difficult for the eavesdropper to find valid information and increasing the security of the system.

(2) Compared with the traditional quantum noise stream encryption, the present invention proposes an improved thickness modulation, which realizes the thickness modulation of the signal in the optical domain by adjusting the optical power (instead of the electrical power). The generated electrical domain signal is synthesized with a high-bit encrypted signal, which makes the number of masking levels more, and uses it to achieve quantum noise stream encryption with higher masking levels, which improves the security performance, that is, enhanced quantum noise stream cipher secure communication Moreover, since the peak-to-peak value of the electrical signal is adjusted by adjusting the power in the optical domain instead of the electrical domain, the modulation depth and the signal-to-noise ratio of the signal are improved, and the communication performance is improved.

(3) Compared with schemes such as quantum key distribution, the present invention hides and transmits key information through a white light interference structure, that is, the key information is hidden in ASE noise, and the ciphertext information is modulated into transmission ciphertext by using the thickness of the optical domain, Since the ciphertext adopts the enhanced quantum noise flow encryption method, it has a very high level number. Then through the optical multiplexing structure, the channel for transmitting the key (hidden channel) and the channel (common channel) for transmitting the ciphertext are coupled into the same fiber, realizing the simultaneous interpretation of the key and ciphertext, that is, the key and ciphertext are in the same fiber. The device structure is simple, which greatly saves the cost of sharing the key between the sender and the receiver.

Description of drawings

Fig. 1 is the schematic diagram of the system principle provided by the present invention;

Fig. 2 is the system structure diagram of the specific implementation case provided by the present invention;

Fig. 3 is the principle schematic diagram of quantum noise stream cipher provided by the present invention;

4 is a schematic diagram of the principle of thickness modulation in the electrical domain based on a single DD-MZM (Dual-Drive Mach-Zehnder Modulator, Dual-Drive Mach-Zehnder Modulator) provided by the present invention;

5 is a level diagram before and after the encryption and equalization of the quantum noise stream at the receiving end provided by the present invention;

6 is a schematic diagram of the white light interference concealed transmission principle provided by the present invention;

FIG. 7 is a signal spectrum diagram of signal concealment transmission provided by the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

As shown in Figure 1, a high-speed secure optical communication system for simultaneous interpretation of key and ciphertext provided by the present invention includes: a concealed signal transmitting module, a public signal transmitting module, a signal transmission module, a concealed signal receiving module, a public signal receiving module and digital signal processing module.

The concealed signal transmitting module uses the noise light source generating unit to generate a wide-spectrum noise optical carrier of linear polarization state, and uses the first Mach-Zehnder interferometer structural unit to modulate the key information encrypted by the quantum noise stream on the phase of the optical carrier, and There is a certain delay difference between the two optical links of the interferometer, so that interference phenomenon cannot occur, and effective information can be concealed. That is, the effective information is tuned on the optical phase and hidden in the broad-spectrum noise as a hidden signal.

The public signal transmitting module uses a narrow linewidth light source to generate a narrow linewidth laser carrier, uses the first modulation unit to modulate the information that needs to be roughly modulated on the intensity of the optical carrier, and uses the second modulation unit to modulate the information that needs to be finely modulated in another In terms of the intensity of one optical carrier, the power adjustment coupling unit is used to realize the proportional adjustment of the two optical powers, and the two optical paths are coupled to realize the optical thickness modulation in the optical domain. That is, the ciphertext information encrypted by modulating the enhanced quantum noise stream on the optical carrier is realized as a public signal.

The signal transmission module performs signal multiplexing, transmission and signal quality compensation through the optical multiplexing unit and the signal transmission and compensation unit.

The concealed signal receiving module makes the received concealed transmitted optical signal enter the second Mach-Zehnder interferometer structural unit, and matches the delay difference of the two channels in the concealed signal transmitting module to within the coherence length of the broad-spectrum noise light source in this structure. It is transmitted to the first photoelectric conversion unit for photoelectric conversion, and the key information used for encryption of the quantum noise stream of the concealed transmission is recovered.

The common signal receiving module uses the optical filter attenuation unit to process the optical signal, transmits it to the second photoelectric conversion unit, and receives the common signal to demodulate and recover the ciphertext.

The digital signal processing module is used to perform digital signal processing on the recovered key information in combination with the recovered ciphertext information, and decrypt to obtain the plaintext.

In order to further illustrate the system of the present invention, a high-speed secure optical communication system with simultaneous interpretation of key and ciphertext provided by the embodiment will now be described in detail.

FIG. 2 is a system structure diagram of a specific implementation case of the present invention.

Furthermore, combined with formula derivation and theoretical model, the realization of enhanced quantum noise stream encryption by the public signal transmitting module is discussed.

The great hidden danger of traditional stream ciphers is that the encrypted ciphertext and the plaintext before encryption are usually signals in the same format. The same is the switch key signal. In this way, the non-cooperating party can obtain the same ciphertext information as the cooperating party, and can obtain the plaintext information by cracking the key through various attack methods such as fast correlation attack. Quantum noise stream ciphers are different from traditional stream ciphers in that they utilize the characteristics of quantum noise to achieve secure signal transmission. Quantum noise includes the spontaneous emission noise of erbium-doped fiber amplifiers and the shot noise of photodetectors, which are inevitably quantum noise. Noise widely exists in various communication systems. Figure 3 shows the schematic diagram of the quantum noise stream cipher. With the help of the key, the quantum noise stream cipher maps the plaintext signal into an ultra-densely modulated signal, so that the Euclidean distance between each level of the signal is very It is far smaller than the scale of quantum noise. Under the cover of quantum noise, non-cooperating parties cannot make correct judgments to obtain ciphertext information, so they cannot use existing attack methods against traditional stream ciphers to attack.

The security performance of quantum noise stream encryption is mainly determined by the number of mask levels, that is, the number of bits of the modulated optical signal. A single intensity modulator is used for pulse amplitude modulation (PAM). The more the pulse amplitude, the more the number of mask levels. Security is higher, and this is determined by the number of bits in the digital-to-analog converter (DAC). For example, the number of mask levels that can be achieved with an 8-digit DAC is 256. In order to overcome the shortage of the number of DAC bits, the method of thickness modulation is mainly used to "add" the number of DAC bits, that is, to "add" multiple low-bit DACs to achieve the effect of high-bit DACs, such as using two 8-bit DACs. The number of masking levels of 2 ¹⁶ can be achieved by "adding" the DAC of the DAC, which greatly improves the security.

The traditional thickness modulation is mainly done in the electrical domain with a single dual-drive Mach-Zehnder modulator (DD-MZM). Figure 4 shows a single DD-MZM, the upper arm inputs M-bit electrical signals, the lower arm inputs N-bit electrical signals, the output light field is assumed to be E ₀ , and the output signal can be expressed as:

Among them, S _coarse and S _fine are the signals of coarse modulation and fine modulation, respectively. In order to make the level interval of the synthesized PAM signal uniform, the signal of coarse and fine modulation must satisfy the following peak-to-peak relationship:

Among them, M and N are the binary digits of the thickness modulation signal respectively. Therefore, in the traditional thickness modulation scheme, the peak-to-peak difference of the electrical signal is very large. As a result, when the peak-to-peak amplitude of one signal is constant, the peak-to-peak value of the other signal is very small, and the signal-to-noise ratio is very low, which leads to the deterioration of communication performance.

In the example of the present invention, the improved thickness modulation structure is shown in the common signal transmitting module in FIG. 2 , the module includes: a laser, a third optical coupler, a second polarization controller, a third polarization controller, and a second modulator , the third modulator, the 90° phase shifter, the first adjustable optical attenuator, and the fourth optical coupler. The narrow linewidth laser output from the laser is divided into two paths and enters the second and third modulators respectively for coarse modulation and fine modulation. The dimming attenuator attenuates the optical power until the optical power of the two parallel links meets a certain ratio requirement, and then outputs the combined circuit through the fourth coupler. For the convenience of analysis, it is assumed that the second and third modulators are both MZM modulators biased at the linear point, and each device is in an ideal state.

The improved coarse and fine modulation uses two parallel MZMs to achieve fine and coarse modulation of the signal. The signals respectively output from a single MZM can be expressed as:

The total signal output is:

Among them, α is the attenuation of optical decay, which is used to modulate the intensity of the optical signal to meet the requirements of the peak-to-peak value of the signal, and j is the generated fixed π/2 phase shift. The light intensity of the signal after passing through the photodetector is:

in,

是在环境比如应力和温度的影响下两个光路产生微小时变光程差导致的附加相移，通常集成器件可以忽略。in,

It is the additional phase shift caused by the tiny time-varying optical path difference between the two optical paths under the influence of environment such as stress and temperature, which can usually be ignored in integrated devices.

Further, in the example of the present invention, as shown in the common signal receiving module in FIG. 2 , the module includes: an optical filter, a second adjustable optical attenuator, a second photodetector, and an oscilloscope. The received optical signal is filtered, the optical power is attenuated, and then enters the photodetector for photoelectric conversion, and then is converted to the digital signal processing module in the oscilloscope for subsequent digital signal processing.

As shown in Figure 5, it is the level diagram before and after equalization at the receiving end after quantum noise stream encryption. It can be seen from the analysis that the encryption effect of the system is better, and the decoding by the legitimate receiver can also obtain a lower bit error rate.

In general, the method adopted in this example to realize enhanced quantum noise flow encryption by adjusting the optical power of the parallel link for optical thickness modulation, has greatly improved security performance and communication performance compared with the traditional scheme.

Furthermore, combined with formula derivation and theoretical model, the realization of white light interference stealth transmission by stealth signal transmitting module and stealth signal receiving module is discussed.

The principle of white light interference concealing transmission information is shown in Figure 6. There are four optical paths from the transmitting end to the receiving end of the broad-spectrum light source: (a) 1-3; (b) 1-4; (c) 2-3; (d) 2-4. According to the optical interference theory, only light within the coherence length can interfere, that is, the interference occurs only when the two optical paths (a) and (d) adjust the delay lines t1 and t2 to make the delay difference within the coherence length of the light source. , demodulate the data loaded on the optical phase on link 2. At the photodetector end, adjust the two delay lines t1 and t2,

When the delay difference between the two channels is within the coherence length, I=2|E| ² +2|E| ² cos(u(t));

When the delay difference between the two paths is beyond the coherence length, I=4|E| ² .

As shown in Figure 2, the concealed signal transmitting module includes: a broad-spectrum noise light source, a polarizer, a first optical coupler, a first polarization controller, a first modulator, a first optical delay, and a second optical coupler . The wide-spectrum light with linear deviation is divided into two paths, one path goes through the phase modulator to increase the optical phase information, and the other path passes through the optical delay device to make the two paths have a certain delay difference, and then the coupler is combined. The light source used in the module is an ASE noise light source with a spectral width of 40 nm. When the spectral shape is approximately rectangular, the coherence length is converted to 60 μm, that is, interference occurs only when the difference in the length of the optical link between the two arms is within 60 μm. Adjust the optical delay device so that the difference between the lengths of the two links exceeds the coherence length, that is, to ensure that no interference will occur in the hidden signal transmitting module.

That is, in the hidden signal transmission module, the key information used for quantum noise stream encryption is loaded on the phase of the broad-spectrum optical noise, and the spectral information after modulation is simulated by the system as shown in Figure 7. The amplitude in the time domain is constant and hidden. In the noise, in the spectral domain, the spectrum of the modulation information is also hidden in the broadband noise spectrum, it is difficult for the eavesdropper to know that there is valid information in it, and the hidden transmission is realized.

As shown in FIG. 2 , the concealed signal receiving module includes: a fifth optical coupler, a second optical delay device, a sixth coupler, a first photodetector, and an oscilloscope. The received optical signal is divided into two paths, one of which is matched and delayed by an optical delay device, so that the delay with the hidden signal transmitter module is kept within the coherence length of the broad-spectrum noise light source, so that interference phenomenon can occur, and the optical phase modulated on the optical phase can be demodulated. Key information used in quantum noise stream encryption.

Only when the optical delay device is adjusted so that the optical path difference between the hidden signal transmitting module and the hidden signal receiving module is within the coherence length of the broad-spectrum light source, interference can occur, and the data loaded on the optical phase on link 1 can be demodulated, and The better the delay matching, the better the demodulation performance.

Further, as shown in FIG. 2 , the signal transmission module includes: a multiplexer, a standard single-mode fiber, a dispersion compensation fiber, an optical amplifier, and a demultiplexing filter. The optical signals of the hidden signal transmitting module and the public signal transmitting module are multiplexed into the standard single-mode fiber for transmission, and the dispersion compensation fiber and the optical amplifier are used for dispersion compensation and power compensation respectively, and then the signal is demultiplexed by the demultiplexing filter. To the hidden signal receiving module and the public signal receiving module. Since the key information encrypted by the quantum noise stream is loaded into the hidden signal transmission module, and the ciphertext information encrypted by the quantum noise stream is loaded into the public signal transmission module, the simultaneous interpretation of the key and ciphertext is realized after multiplexing. The key information is loaded on the phase of the broadband noise light, so that the secret key information is difficult to be discovered by the eavesdropper. If the delay of the hidden signal transmitting module and the hidden signal receiving module is fixed to match, the system does not need extra expensive equipment and other The key distribution is carried out by the optical path, which greatly saves the cost.

According to the above content, a high-speed secure optical communication system with simultaneous interpretation of key and ciphertext described in the present invention can be constructed. The bit error rate obtained by the digital signal processing at the end can reach below the FEC (Forward Error Correction, forward error correction code) decision line, that is, the demodulation performance of the legitimate receiver is good.

Those skilled in the art can easily understand that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, etc., All should be included within the protection scope of the present invention.

Claims (10)

1. A high-speed secure optical communication system for key ciphertext concurrent transmission, the system comprising: the device comprises a transmitting end, a signal transmission module and a receiving end;

the transmitting end includes: the device comprises a hidden signal transmitting module and a public signal transmitting module; the receiving end comprises a hidden signal receiving module, a public signal receiving module and a digital signal processing module;

the secret signal transmitting module is used for modulating the secret key information electric signal encrypted by the quantum noise flow to a wide-spectrum noise optical carrier, transmitting the modulated optical signal to the signal transmission module as a secret signal, and the spectral width of the wide-spectrum noise optical carrier is higher than 10 nm;

the public signal transmitting module is used for modulating the ciphertext information electric signal encrypted by the quantum noise flow to a narrow linewidth optical carrier, and transmitting the modulated optical signal to the signal transmitting module as a public signal, wherein the linewidth of the narrow linewidth optical carrier is lower than 100 MHz;

the signal transmission module is used for respectively transmitting the hidden signal and the public signal to the hidden signal receiving module and the public signal receiving module of the receiving end;

the secret signal receiving module is used for receiving the secret signal, carrying out photoelectric conversion and recovering key information used for quantum noise stream encryption;

the public signal receiving module is used for receiving public signals, carrying out photoelectric conversion and recovering ciphertext information;

and the digital signal processing module is used for carrying out digital signal processing on the recovered key information and the recovered ciphertext information and decrypting to obtain a plaintext.

2. The system of claim 1, wherein the common signal transmitting module comprises:

the narrow linewidth light source is used for generating narrow linewidth laser, and transmitting the narrow linewidth laser to the first intensity modulation unit and the second intensity modulation unit respectively after shunting;

the first intensity modulation unit is used for carrying out intensity modulation on M bits before the PAM electrical signal on one path of optical carrier and transmitting the optical signal after coarse modulation to the power regulation unit;

the second intensity modulation unit is used for carrying out intensity modulation on N bits after the PAM electrical signal is carried out on the other path of optical carrier, and transmitting the finely modulated optical signal to the power regulation unit;

and the power adjusting unit is used for adjusting the power proportion of the two paths of optical signals and coupling the two optical paths, so that the adjusted coupled optical signal is a uniform M + N-bit PAM optical signal.

3. The system of claim 1, wherein the common signal transmitting module comprises in particular the following components:

a laser for emitting a light carrier to the third optical coupler;

the third optical coupler is used for branching the optical carrier, one path of the optical carrier is transmitted to the second polarization controller, and the other path of the optical carrier is transmitted to the third polarization controller;

the second polarization controller is used for adjusting the polarization state of the optical carrier to keep consistent with the polarization main shaft of the crystal in the second modulator and transmitting the adjusted optical carrier to the second modulator;

the third polarization controller is used for adjusting the polarization state of the optical carrier to keep consistent with the polarization main shaft of the crystal in the third modulator and transmitting the adjusted optical carrier to the third modulator;

the second intensity modulator is used for modulating the roughly modulated electrical signal data onto an optical carrier and transmitting the modulated optical signal to the 90-degree phase shifter;

the third intensity modulator is used for modulating the fine modulation electric signal data to an optical carrier and transmitting the modulated optical signal to the first variable optical attenuator;

a 90 ° phase shifter for performing a fixed pi/2 phase shift on the received coarse modulated optical signal and transmitting the optical signal to a fourth optical coupler;

the first variable optical attenuator is used for attenuating the fine modulation optical signal and transmitting the fine modulation optical signal to the fourth optical coupler;

and the fourth optical coupler is used for combining the optical signals and transmitting the optical signals to the multiplexer.

4. The system of claim 3, wherein the second and third intensity modulators are Mach-Zehnder modulators for modulating an electrical signal onto an optical signal amplitude, and the Mach-Zehnder modulators operate in a linear region of a modulation profile.

5. The system as claimed in claim 1, wherein the signal transmission module multiplexes the hidden signal and the common signal into the same optical fiber for transmission, and demultiplexes two signals at a receiving end into the hidden signal receiving module and the common signal receiving module.

6. The system of claim 5, wherein the signal transmission module specifically comprises the following components:

the multiplexer is used for multiplexing the hidden signal and the public signal and transmitting the multiplexed optical signal to a standard single-mode optical fiber;

the standard single-mode optical fiber is used as a transmission channel of an optical signal and transmits the optical signal to the dispersion compensation optical fiber;

the dispersion compensation optical fiber is used for compensating dispersion generated by transmitting an optical signal in the optical fiber, and the compensated optical signal is transmitted to the optical amplifier;

the optical amplifier is used for compensating the optical power loss of the optical signal in the transmission process of the optical fiber, and the compensated optical signal is transmitted to the demultiplexing filter;

and the demultiplexing filter is used for demultiplexing the hidden optical signal and the public optical signal and transmitting the demultiplexed optical signals to the hidden signal receiving module and the public signal receiving module.

7. The system of claim 5, wherein the signal transmission module implements multiplexing through a wavelength division multiplexing device or an optical coupler.

8. The system of any of claims 1 to 7, wherein the suppressed signal transmission module comprises:

the noise light source generating unit is used for generating a wide-spectrum noise light carrier wave in a linear polarization state and transmitting the wide-spectrum noise light carrier wave to the first Mach-Zehnder interferometer structural unit;

and the first Mach-Zehnder interferometer structural unit is used for modulating the secret key information encrypted by the quantum noise stream on the phase of the optical carrier.

9. The system of any of claims 1 to 7, wherein the suppressed signal receiving module comprises:

the second Mach-Zehnder interferometer structural unit is used for matching two paths of delay differences in the hidden signal transmitting module to be within the coherence length of the wide-spectrum noise light source and transmitting the delay differences to the first photoelectric conversion unit;

and the first photoelectric conversion unit is used for converting the optical signal into an electric signal, recovering key information used for encrypting the quantum noise flow which is transmitted in a hiding way, and transmitting the key information to the digital signal processing module.

10. The system of any one of claims 1 to 7, wherein the common signal receiving module comprises:

the optical filtering attenuation unit is used for filtering out-of-band noise of the demultiplexed public signal, and transmitting the optical power of the attenuator to the second photoelectric conversion unit after the optical power is received by the second photoelectric conversion unit;

and the second photoelectric conversion unit is used for receiving the filtered and attenuated public signal, demodulating a recovery ciphertext and transmitting the recovery ciphertext to the digital signal processing module.

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