Abstract
A coherent detection-based high-speed chaotic secure transmission method includes: at a transmit terminal in a chaotic secure transmission system, optically coupling an optical chaotic carrier and transmission information by using an orthogonal basis to mask the transmission information by using a noise-like feature of the chaotic carrier, so as to obtain a chaotic masked signal; adding a fast phase disturbance and a fast polarization disturbance to the chaotic masked signal and transmitting the chaotic masked signal over an optical fiber transmission link; and at a receive terminal, obtaining the chaotic masked signal through coherent detection, compensating the chaotic masked signal for linear and nonlinear effects through digital signal processing, and using a polarization orthogonal basis- or phase orthogonal basis-based chaotic decryption algorithm to separate the chaotic carrier from the signal so as to complete decryption.
Claims
1. A coherent detection-based high-speed chaotic secure transmission method, comprising the following steps: step 1: at a transmit terminal in a chaotic secure transmission system, optically coupling an optical chaotic carrier c(t) and transmission information m(t) by using an orthogonal basis, masking the transmission information by using a noise-like feature of the optical chaotic carrier to obtain a chaotic masked signal cm(t); wherein the orthogonal basis comprises a polarization orthogonal basis and a phase orthogonal basis; step 2: adding a fast phase disturbance and a fast polarization disturbance to the chaotic masked signal cm(t) to improve security of the chaotic masked signal; and transmitting the chaotic masked signal over an optical fiber transmission link; and step 3: at a receive terminal in the chaotic secure transmission system, obtaining the chaotic masked signal with intensity, phase, and polarization information through coherent detection; and compensating the chaotic masked signal after the coherent detection for linear and nonlinear effects through digital signal processing and using a polarization orthogonal basis-based chaotic decryption algorithm or a phase orthogonal basis-based chaotic decryption algorithm to separate the optical chaotic carrier from the chaotic masked signal so as to complete decryption.
2. The coherent detection-based high-speed chaotic secure transmission method according to claim 1, wherein using the polarization orthogonal basis-based chaotic decryption algorithm to decrypt the chaotic masked signal obtained by optically coupling the optical chaotic carrier c(t) and the transmission information m(t) by using the polarization orthogonal basis is specifically implemented as follows: (1) representing polarization rotation in the optical fiber transmission link by an azimuth angle θ and an ellipticity angle φ, and setting a test range of the azimuth angle θ and the ellipticity angle φ as follows: wherein B represents a total number of test angles, and the test range of the azimuth angle θ and the ellipticity angle φ is from −90° to 90°; (2) assuming that each set of test angles is composed of θ.sub.k and φ.sub.m, and a value range of k and m is 0, 1, 2, . . . , and B−1; performing polarization tracking on the chaotic masked signal by using an inverse transmission matrix M.sup.−1 to implement chaotic decryption; wherein the inverse transmission matrix M.sup.−1 is expressed as follows: the polarization tracking process is expressed as follows: wherein E.sub.inx and E.sub.iny represent chaotic masked signals after the compensation for the linear and nonlinear effects; and E.sub.outx,k,m and E.sub.outy,k,m represent chaotic masked signals after the polarization rotation is performed on the test angles θ.sub.k and φ.sub.m by using the inverse transmission matrix; (3) using two-level coarse and fine steps to select test values of the two angles from −90° to 90°; wherein, a first-level coarse step of 18° is used to select angles within the test range, E.sub.outx,k,m and E.sub.outy,k,m are obtained, and a Godard's error is introduced to analyze signal quality and determine whether the polarization tracking decrypts the chaotic masked signal; tests are conducted for all test angle combinations, and an optimal test angle combination (θ.sub.subopt, φ.sub.subopt) corresponding to a smallest Godard's error is found; the optimal test angle combination (θ.sub.subopt, φ.sub.subopt) is used to establish test ranges (θ.sub.subopt−δ.sub.1, φ.sub.subopt+δ.sub.1) and (φ.sub.subopt−δ.sub.1, φ.sub.subopt+δ.sub.1) with a second-level fine step; gi represents the second-level fine step having a size of 3°, and tests are conducted for all test angle combinations, and an optimal test angle combination (θ.sub.opt, φ.sub.opt) corresponding to a smallest Godard's error is found to demodulate the chaotic masked signal; and (4) defining the Godard's error of the polarization orthogonal basis-based chaotic decryption algorithm as follows: wherein |E.sub.outx/y,k,m|.sup.2 represents intensity of the signals E.sub.outx,k,m and E.sub.outy,k,m, N represents a number of sample points of the signal, and RP.sub.outx,k,m represents constant power of the signals E.sub.outx,k,m and E.sub.outy,k,m.
3. The coherent detection-based high-speed chaotic secure transmission method according to claim 2, wherein the step size in the polarization orthogonal basis-based chaotic decryption algorithm is adjusted based on an actual situation or obtained by adopting an adaptive step size.
4. The coherent detection-based high-speed chaotic secure transmission method according to claim 1, wherein using the phase orthogonal basis-based chaotic decryption algorithm to decrypt the chaotic masked signal obtained by optically coupling the optical chaotic carrier c(t) and the transmission information m(t) by using the phase orthogonal basis is specifically implemented as follows: (1) first compensating the chaotic masked signal received by the receive terminal for the linear and nonlinear effects to obtain a compensated chaotic masked signal, wherein the compensated chaotic masked signal is represented by E.sub.inp; then performing distributed Fourier transform on the compensated chaotic masked signal E.sub.inp to obtain spectrum information of the compensated chaotic masked signal and searching for a maximum peak of the spectrum information to obtain initial frequency offset information f.sub.c of the compensated chaotic masked signal; and finally, performing initial frequency offset compensation to obtain a signal E.sub.inc, wherein a specific process is expressed as follows:
E.sub.inc=E.sub.inp exp{−j2π max(|FFT(E.sub.inp)|)t}=E.sub.in exp(−j2πf.sub.ct) (5) (2) performing serial-to-parallel conversion on the signal E.sub.inc after the initial frequency offset compensation to obtain a series of parallel data, and testing a phase slope l.sub.f of the series of parallel data to further estimate an accurate frequency offset of the signal, so as to implement accurate frequency offset compensation of the signal; using an average information phase of parallel data having signal intensity greater than a threshold R.sub.th to compensate for a laser phase noise disturbance; and obtaining the transmission information through in-phase and quadrature (IQ) separation after the compensation to implement chaotic decryption.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a block diagram of a coherent detection-based high-speed chaotic secure transmission method according to the present disclosure;
[0027] FIG. 2 is a block diagram of a polarization orthogonal basis-based chaotic decryption algorithm according to the present disclosure;
[0028] FIG. 3 is a block diagram of a phase orthogonal basis-based chaotic decryption algorithm according to the present disclosure;
[0029] FIG. 4A shows bit error rates of a 56 Gbit/s quadrature phase shift keying (QPSK) signal (a) before and after decryption at different masking rates and FIG. 4B shows bit error rates of a 112 Gbit/s 16-quadrature amplitude modulation (QAM) signal (b) before and after decryption at different masking rates; and
[0030] FIG. 5A shows bit error rates of a 10 Gbit/s OOK signal (a) before and after decryption over optical fibers of different lengths and FIG. 5B shows bit error rates of a 28 Gbit/s OOK signal (b) before and after decryption over optical fibers of different lengths.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0031] The present disclosure is further described in detail below with reference to the accompanying drawings and specific embodiments.
[0032] A coherent detection-based high-speed chaotic secure transmission method in the present disclosure includes the following steps:
[0033] Step 1: At a transmit terminal in a chaotic secure transmission system, optically couple an optical chaotic carrier c(t) and transmission information m(t) by using an orthogonal basis (which may be a polarization orthogonal basis or phase orthogonal basis, but is not limited thereto) to mask the transmission information by using a noise-like feature of the chaotic carrier, so as to obtain a chaotic masked signal cm(t).
[0034] Step 2: Add a fast phase disturbance and a fast polarization disturbance to the chaotic masked signal cm(t) to improve security of the chaotic masked signal; and transmit the chaotic masked signal over an optical fiber transmission link.
[0035] Step 3: The chaotic masked signal transmitted over the optical fiber may be subject to various linear and nonlinear damages, such as fiber dispersion, polarization mode dispersion, and nonlinear effects. Therefore, at a receive terminal in the chaotic secure transmission system, obtain the chaotic masked signal with intensity, phase, and polarization information through coherent detection; and compensate the chaotic masked signal after the coherent detection for linear and nonlinear effects through digital signal processing and use a polarization orthogonal basis- or phase orthogonal basis-based chaotic decryption algorithm to separate the chaotic carrier from the signal so as to complete decryption.
[0036] As shown in FIG. 1, in the present disclosure, information modulation modules (101.sub.1 to 101.sub.N) modulate amplitude modulation OOK or m-PAM, phase modulation m-PSK, or m-QAM signals; chaotic carrier generation modules (102.sub.1 to 102.sub.N) generate optical chaotic carriers; orthogonal basis multiplexing modules (103.sub.1 to 103.sub.N) optically couple the optical chaotic carriers and the signals by using the orthogonal basis to obtain chaotic masked signals; the information modulation modules (101.sub.1 to 101.sub.N), the chaotic carrier generation modules (102.sub.1 to 102.sub.N), and the orthogonal basis multiplexing modules (103.sub.1 to 103.sub.N) form transmitters (104.sub.1 to 104.sub.N) for one or N channels of wavelengths (services) in the system; a wavelength division multiplexer (105) couples the to-be-transmitted signals of a plurality of wavelengths (services), and a rapid deflection apparatus (106) rapidly deflects the chaotic masked signals; the signals are transmitted over one or N segments of optical fibers (107.sub.1 and 107.sub.N), and one or N optical amplifiers (108.sub.1 to 108.sub.N) compensate the signals for corresponding transmission losses; a wavelength division demultiplexer (109) separates the transmitted chaotic masked signals of the plurality of wavelengths, and a coherent receiver (110.sub.1 to 110.sub.N) performs analog-to-digital conversion on the signals to obtain digital signals; and finally, digital signal processing modules (111.sub.1 to 111.sub.N) perform corresponding signal damage compensation and chaotic signal decryption.
[0037] FIG. 2 is a block diagram of a polarization orthogonal basis-based chaotic decryption algorithm. The digital signal processing modules (111.sub.1 to 111.sub.N) perform dispersion and nonlinear effect compensation on the received chaotic masked digital signals. For the compensated chaotic masked data, different combinations of test azimuth angles θ.sub.k and test ellipticity angles φ.sub.m are obtained. Then, an inverse transmission matrix M.sup.−1 is used to perform polarization tracking on the chaotic masked signals. Chaotic masked signals E.sub.outx,k,m and E.sub.outy,k,m obtained after the polarization tracking are used to calculate a Godard's error, and it is determined based on the error whether the polarization tracking decrypts the chaotic masked signals. Tests are conducted for all test angle combinations, and a smallest Godard's error is found. A test angle combination corresponding to the error is an optimal test angle combination (θ.sub.opt, φ.sub.opt). This test angle combination can be used to decrypt the chaotic masked signals. Then, laser phase noise is compensated through carrier phase retrieval. Finally, a signal demodulation module demodulates the signals.
[0038] FIG. 3 is a block diagram of a phase orthogonal basis-based chaotic decryption algorithm according to the present disclosure. The digital signal processing modules (111.sub.1 to 111.sub.N) perform dispersion and nonlinear effect compensation on the received chaotic masked digital signals. First, distributed Fourier transform is performed on compensated chaotic masked data E.sub.inp to obtain spectrum information of the signals. Then, initial frequency offset information f.sub.c of the signals is obtained by searching for a maximum peak of the spectrum. Finally, initial frequency offset compensation is performed. Serial-to-parallel conversion is performed on signals E.sub.inc after the initial frequency offset compensation to obtain a series of parallel data. Phase slopes l.sub.f of the parallel data are tested to further estimate accurate frequency offsets of the signals, so as to implement accurate frequency offset compensation of the signals. Then, an average information phase of parallel data whose signal intensity is greater than a threshold R.sub.th is used to compensate for a laser phase noise disturbance. After the compensation, transmission information can be obtained through IQ separation to implement chaotic decryption.
[0039] FIGS. 4A-4B show performance tests of using a polarization orthogonal basis-based chaotic decryption algorithm to decrypt chaotic masked signals obtained through coupling by using a polarization orthogonal basis in the present disclosure. FIGS. 4A-4B compare bit error rates of a 56 Gbit/s QPSK chaotic masked signal (a) and a 112 Gbit/s 16-QAM chaotic masked signal (b) before decryption with those after decryption at different masking rates. The masking rate is a ratio of an amplitude of information to an amplitude of a chaotic carrier. It can be learned from the two diagrams that the bit error rates before decryption at all masking rates are higher than 0.3. Therefore, it can be concluded that the chaotic carrier can successfully hide the signal in these two systems, and it is very difficult for an eavesdropper to directly obtain the transmission information. By using the algorithm in the present disclosure, after the 56 Gbit/s QPSK chaotic masked signal is transmitted over 2000 km, demodulation performance is still lower than a 7% forward error correction (FEC) threshold (corresponding to a bit error rate 3.8×10.sup.−3). After the 112 Gbit/s 16-QAM chaotic masked signal is transmitted over 1040 km, a demodulation error bit rate of the algorithm can be lower than 2.4×10.sup.−3 (corresponding to a 20% FEC threshold). It is also learned that decryption performance becomes worse as the masking rate decreases. This is mainly because a residual of chaotic cancellation is equivalent to large noise power, which directly affects the decryption performance of the algorithm in the present disclosure to some extent.
[0040] FIGS. 5A-5B show performance tests of using a phase orthogonal basis-based chaotic decryption algorithm to decrypt chaotic masked signals obtained through coupling by using a phase orthogonal basis in the present disclosure. FIGS. 5A-5B compare bit error rates of a 10 Gbit/s OOK chaotic masked signal (a) and a 28 Gbit/s OOK chaotic masked signal (b) before decryption with those after decryption over optical fibers of different lengths. It can be learned from the two diagrams that the bit error rates before decryption are higher than 0.1. It can be considered that without using the chaotic decryption algorithm in the present disclosure, it is very difficult for an eavesdropper to directly obtain the transmission information. When decryption performance is lower than the 20% FEC threshold, the 10 Gbit/s OOK chaotic masked signal and the 28 Gbit/s OOK chaotic masking signal (b) can be transmitted over 2000-km and 1440-km optical fibers, respectively. It can be observed that the decryption performance of the 28 Gbit/s OOK chaotic masked signal is much worse than that of the 10 Gbit/s OOK chaotic masked signal. This is mainly because a signal at a higher transmission rate is more sensitive to damage in an optical fiber link and more prone to damage.
