Apparatus and method for pilot tone-assisted demodulation in coherent optical communication systems

Abstract

A pilot tone-assisted demodulation method is provided for coherent optical communication systems, particularly continuous-quantum quantum key distribution (CV-QKD) systems, which enables post-signal acquisition compensation for random phase fluctuations of the laser used by these systems, thereby easily, efficiently, and cost-effectively solving the frequency lock problem.

Description

[Technical Field]

[0001] The present invention belongs to the field of pilot tone-assisted demodulation for coherent optical communication systems, specifically to the field of signal demodulation and frequency-locking techniques, which are assisted by transmitting a pilot tone along with the data signal, which is used as a phase reference for the transmitter laser.

[0002] In particular, the present invention relates to an apparatus and method for assisted demodulation in a continuous-quantum quantum key distribution (CV-QKD) system, which is a cryptographic technique based on the principles of quantum mechanics for securing communication channels. The present invention makes it possible to compensate for random phase fluctuations of the laser used in these systems after the signal has been acquired, thereby solving the frequency lock problem simply, efficiently, and cost-effectively. [Background technology]

[0003] The introduction of pilot tones in combination with other techniques such as temporal synchronization and low-complexity implementations into the field of quantum key distribution is a relatively recent development. Generally, this technique is used to implement a system known as a PLL (Phase-Locked Loop), a method used in classical communications before the advent of quantum key distribution (QKD), with the aim of frequency stabilizing two laser sources before acquiring a received signal.

[0004] While there are some documents discussing methods for correcting phase drift after signal acquisition, which fall within the scope of this invention, none of them are similar to this invention. Some proposals include phase compensation through statistical analysis or signal demodulation using phase information extracted from emitter frequency measurements.

[0005] Therefore, it has been found that the current state of the art includes both methods of performing correction before data acquisition and after data acquisition to correct the phase drift of two lasers. However, for a CV-QKD system that compensates for the random phase fluctuations of the lasers used in the CV-QKD system after signal acquisition, as well as for solving the frequency locking problem similar to that presented here and achieving a similar degree of flexibility and simplicity, no example of a literature has been found. Summary of the Invention Problems to be Solved by the Invention

[0006] An object of the present invention is an apparatus and method for pilot tone assisted demodulation in a coherent optical communication system, particularly for a CV-QKD (continuous variable quantum key distribution) system. This apparatus and method can compensate for the random phase fluctuations of the lasers used in these systems after signal acquisition, thereby solving the frequency locking problem simply, efficiently, and cost-effectively. Means for Solving the Problems

[0007] To implement this method, only a signal mixer and a filter are used, thereby reducing the complexity and cost of the apparatus compared to a PLL loop typically used to stabilize the laser frequency. The flexibility of this technique lies in the ability of this technique to be implemented digitally using a software-defined radio FPGA or analogously using hardware with an electronic circuit.

[0008] Regarding an application example of the present invention, the present invention is applicable to any coherent optical communication system based on the transmission of a pilot tone as a phase stabilization method, generally a CV-QKD system.

[0009] The method includes the following steps. 1. In a receiver, a signal is measured using a coherent optical detector that can be heterodyne or homodyne. At this time, the output of the coherent detector is an analog electronic signal consisting of a data band and a pilot tone.

[0010] 2. The signal is split and processed through multiple paths. The electronic signal V(t) at the output in the case of a homodyne detector and the two electronic signals V I ’(t) and V Q ’(t) in the case of a heterodyne detector both have similar frequency spectra. These signals can be mathematically expressed as follows.

[0011] V I ’(t)=V1cos(2πΔ f t + Δ Φ )V I (t)+V2cos(2π(Δ f + f p )t + Δ Φ ), V Q ’(t)=V1sin(2πΔ f t + Δ Φ )V Q (t)+V2sin(2π(Δ f + f p )t + Δ Φ ), V(t)=V1[cos(2πΔ f t + Δ Φ )V I (t)+sin(2πΔ f t + Δ Φ )V Q (t)]+V2cos(2π(Δ f + f p )t + Δ Φ ) In the above formula, V1 and V2 are the amplitudes of the data band and the pilot tone, respectively, Δ f is the frequency difference between the two lasers, Δ Φ is the phase difference between the two lasers, V I(t) is a modulated signal transmitted by the transmitter in a quadrature of I, and V Q (t) is the signal transmitted in a phase perpendicular to Q (Q quadrature).

[0012] From here on, the method for heterodyne detectors will be described, as it is redundantly the same as the method for homodyne detectors. Therefore, the signal processing substeps are as follows:

[0013] a. Signal V I '(t) and V Q '(t) is the same frequency f as the pilot tone in the first mixer. p It is mixed with another electronic signal. The spectrum of the output of any mixer consists of the lower and upper sidebands of the product of the local oscillator signal, with the local oscillator signal acting as the carrier signal.

[0014] b. In the previous step, at the output stage of the mixer, only the lower sideband is filtered out using a band-pass filter from the two resulting bands, specifically, frequency Δ f +f p -f p =Δ f In other words, only the pilot tone shifted to the lower sideband, which has the same frequency as the frequency difference between the two lasers including all random drift, is filtered using a band-pass filter. This signal is V I '(t) or V Q '(t) = P(t) = cos(2πf p The result of multiplying the signal represented by t) can be expressed mathematically. In either case, the process is the same, and therefore only the first one is considered. The result of this multiplication is as follows:

[0015]

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[0016] After the bandpass filter, the first three terms cancel each other out, leaving only the signal necessary to demodulate the received signal.

[0017]

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[0018] Only that remains. c. Next, the original signal V I '(t) is again mixed with the newly obtained demodulated signal in the third mixer (or the fourth mixer in the case of a phased path). This signal is then mixed with the received signal V I '(t) can be analytically calculated by multiplying it by the signal D(t) at the output of the band-pass filter, and the result is as follows:

[0019]

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[0020] d. When this signal is filtered using a low-pass filter, only the data band containing the transmitted signal is extracted. After this low-pass filter, only the second term remains, and therefore the result is the signal V sent out by the transmitter. I It is proportional to (t).

[0021]

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[0022] To supplement the description given and to aid in a better understanding of the features of the present invention, a set of drawings is provided as an integral part of this description, according to a preferred example of a practical embodiment of the present invention. These drawings, which are illustrative and non-limiting, depict the following: [Brief explanation of the drawing]

[0023] [Figure 1]This is a diagram of the apparatus of the present invention for a homodyne coherent detector. [Figure 2] This is an operational diagram of the present invention in the case of a heterodyne coherent detector. [Modes for carrying out the invention]

[0024] Preferred embodiments of the apparatus and pilot tone-assisted demodulation method in a communication system are described below with reference to Figure 1 for a homodyne coherent detector and Figure 2 for a heterodyne coherent detector.

[0025] First, Figure 1 shows a diagram of the apparatus for a homodyne coherent detector. In this case, since it is a homodyne detector, there is only one input V(t). In the method of the present invention for this embodiment, a frequency f equal to the frequency of the pilot tone is used. p The signal (1) is generated. This signal is supplied to both the first mixer (5) and the second mixer (5), which are applied to the portions corresponding to the I perpendicular phase and Q perpendicular phase, respectively, passing through a 90° phase delay network (2) before being input to the second mixer.

[0026] The remaining inputs of both mixers (5) are supplied with the input signal V(t). When the outputs corresponding to each of these mixers (5) pass through the band-pass filter (3), the required demodulation signal is obtained.

[0027] After exiting the filter (3), the resulting signals are supplied to the third mixer (5) and the fourth mixer (5), respectively, while the original input signal V(t) is supplied again to the remaining inputs of both mixers (5).

[0028] When both output signals from these mixers (5) pass through their respective low-pass filters (4), the desired V I (t) Signal and V Q (t) signal is obtained. Next, Figure 2 shows an embodiment of a device for a heterodyne coherent detector, which uses two signals V I '(t) and V Q We obtain (t). At the same time, we obtain a frequency f equal to the frequency of the pilot tone. p The signal (1) is generated in the pilot tone signal generator (1). This signal is supplied to the first mixer (5) and the second mixer (5), which are applied to the portions corresponding to the I perpendicular phase and Q perpendicular phase, respectively, without correcting their phases.

[0029] In the remaining input section of both mixers (5), the input signal V I '(t) and V Q (t) is supplied. The outputs corresponding to each of these mixers (5) pass through the band-pass filter (3) to obtain the required demodulation signal.

[0030] After exiting the bandpass filter (3), their outputs are fed to the third mixer (5) and the fourth mixer (5), respectively, while the remaining inputs of both mixers (5) receive the corresponding original input signals V I '(t) and V Q '(t) is supplied again.

[0031] When both output signals from these mixers (5) pass through their respective low-pass filters (4), the desired V I (t) Signal and V Q (t) signal is obtained.

Claims

1. - In a pilot tone assist demodulation device in a communication system equipped with a heterodyne coherent detector, - Pilot tone, first signal V Ｉ '(t), and the second signal V Ｑ A heterodyne coherent optical detector obtains (t), - Frequency f equal to the pilot tone frequency ｐ A pilot tone generator (1) generates the signal, - The pilot tone generator (1) and the first signal V Ｉ The first mixer (5) connected to (t), - The pilot tone generator (1) and the second signal V Ｑ The second mixer (5) connected to (t), - A first band-pass filter (3) connected to the first mixer (5) and a second band-pass filter (3) connected to the second mixer (5) for obtaining a demodulation signal, - The first band-pass filter (3) and the first signal V Ｉ A third mixer (5) connected to (t), - The second band-pass filter (3) and the second signal V Ｑ The fourth mixer (5) connected to (t), - A first low-pass filter (4) and a second low-pass filter (4) are connected to the third mixer (5) and the fourth mixer (5), respectively. A pilot tone assist demodulation device in a communication system equipped with a heterodyne coherent detector, characterized by comprising the following:

2. - In a pilot tone assist demodulation device in a communication system equipped with a homodyne coherent detector, - A homodyne coherent photodetector for obtaining a pilot tone and signal V(t), - Frequency f equal to the pilot tone frequency ｐ A pilot tone generator (1) generates the signal, - The pilot tone generator (1) and the first mixer (5) connected to the signal V(t) are: - A phase delay network (2) connected to the pilot tone generator (1), - A second mixer (5) connected to the phase delay network (2) and the signal V(t), - A first band-pass filter (3) connected to the first mixer (5) and a second band-pass filter (3) connected to the second mixer (5) for obtaining a demodulation signal, - A third mixer (5) is connected to the first band-pass filter (3) and to the signal V(t), - A fourth mixer (5) is connected to the second band-pass filter (3) and to the signal V(t), - A first low-pass filter (4) and a second low-pass filter (4) are connected to the third mixer (5) and the fourth mixer (5), respectively. A pilot tone assist demodulation device in a communication system equipped with a homodyne coherent detector, characterized by comprising:

3. - In a method for pilot tone-assisted demodulation in a coherent optical communication system, - A step of capturing the demodulated signal using a coherent optical detector to obtain a signal V(t) consisting of a pilot tone and a data band, - Mixing the signal V(t) with an electronic signal having the same frequency f as the pilot tone, resulting in a signal having a spectrum including a lower sideband and an upper sideband; ｐ and obtaining a signal having a spectrum including a lower sideband and an upper sideband as a result; - The electronic signal f shifted to the lower sideband ｐ The steps include obtaining a demodulation signal by filtering the signal, - The steps of mixing the signal V(t) with the demodulation signal to obtain a demodulated signal, - The step of filtering the demodulated signal to obtain a signal of interest defined by the data band. A method for pilot tone-assisted demodulation in a coherent optical communication system, characterized by including the following:

4. - The coherent optical detector is a homodyne detector, and the method is - Same frequency f as the pilot tone mentioned above ｐ An additional step of phase-shifting the aforementioned electronic signal by 90° before it is mixed with the signal V(t) The method according to claim 3, further comprising:

5. - The coherent photodetector is heterodyne, and the method is - An additional step of applying each of the above steps to the first output signal of the heterodyne sensor, - An additional step of applying each of the above steps to the second output signal of the heterodyne sensor: The method according to claim 3, further comprising:

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