A passive phase-stabilized radio frequency optical transmission system

Abstract

A passive phase-stable radio frequency optical transmission system modulates the signal V0 to be transmitted onto an optical carrier with a wavelength of λ1 at the near end. The signal is coupled to an optical circulator 1, passed through an optical beam splitter, and transmitted to n remote ends. The signal is received by an optical circulator 2, passed through a wavelength division multiplexer, and input from an upper branch to a photodetector 1 for demodulation, power division, and filtering to obtain signals V1 and V2. Each remote end modulates V1 onto an optical carrier with a wavelength of λn+1, inputs the optical circulator 2, and transmits the signal back to the near end. The signal passes through an optical beam splitter and the optical circulator 1, and then inputs an erbium-doped fiber amplifier. The signal is coupled with the modulated signal of the λ1 optical carrier and transmitted to the remote end again. The signal passes through a wavelength division multiplexer and inputs from a lower branch to a photodetector 2 for demodulation to obtain a signal V3, which is mixed with V2 and filtered to obtain a phase-stable signal V4.

Description

Technical Field

[0001] The present invention belongs to the field of optical transmission technology, and in particular relates to a frequency mixing phase stabilization technology. Background Art

[0002] High stability in RF signal transmission and distribution is crucial for applications such as distributed coherent radar and high-precision clock local oscillator distribution. Optical fiber, due to its light weight, low transmission loss, and resistance to electromagnetic interference, is gradually replacing cable transmission for long-distance RF transmission in multi-antenna systems.

[0003] Environmental disturbances such as temperature and stress, as well as factors such as the optical fiber's own dispersion and coherent Rayleigh scattering, can cause jitter in the signal transmission phase and reduce system stability.

[0004] Traditional distributed phase-stable transmission technology uses a phase-locked loop (PLL) for active phase stabilization. A phase detector extracts phase jitter information from the round-trip signal, which is then used to provide feedback control for the phase and delay compensator to achieve signal phase stabilization. However, limitations of the phase detector include a limited phase stability range, slow response due to multiple feedback loops, and increased system complexity due to the phase detection module and compensation drive circuit.

[0005] Passive phase stabilization using conjugate mixing incorporates link phase jitter information into the transmitted signal through mixing, achieving phase compensation. Compared to active phase stabilization, mixing phase stabilization utilizes the principle of phase conjugate cancellation. The system architecture is simple and easy to engineer. Theoretically, it has an infinite delay compensation range, eliminates the need for feedback control and adjustable components, and offers faster response times.

[0006] Passive phase stabilization has the following problems: multi-stage mixing introduces insertion loss and additional phase noise, the spurious signals introduced by the mixer interfere with the useful signal, and distributed phase stabilization architecture has rarely been studied. Summary of the Invention

[0007] In order to solve the technical problems existing in passive phase stabilization, the present invention proposes a passive phase-stabilized RF optical transmission system, which adopts the technical solution of conjugate mixing phase-stabilized transmission architecture, and produces the technical effects of single-stage mixing phase stabilization, near-end optical multiplexing transmission, and doubled frequency output.

[0008] The system includes a near-end and a far-end. The near-end includes a continuous wave laser, an electro-optical intensity modulator and an optical circulator. The far-end system includes a second electro-optical intensity modulator, a second continuous wave laser, a second optical circulator, a wavelength division multiplexer, two photodetectors, an electric power splitter, three bandpass filters and a conjugate mixer.

[0009] Continuous wave laser 1 generates an optical carrier with a wavelength of λ1, which, together with the signal V0 to be transmitted, is input into electro-optical intensity modulator 1, which modulates the signal V0 to be transmitted onto the λ1 optical carrier. The optical carrier is then input into optical circulator 1, which specifies the transmission path of the optical signal and transmits it to the remote end.

[0010] Optical circulator 2 receives the optical signal, passes through the wavelength division multiplexer, and outputs the optical carrier with a wavelength of λ1 from the upper branch. The RF signal is demodulated by photodetector 1 and input into the electric power splitter, which splits the demodulated signal into two paths with equal power. The demodulated signals are filtered out by bandpass filter 1 and bandpass filter 3 respectively to obtain the demodulated signals V1 and V2 of the specified frequency.

[0011] Continuous wave laser 2 generates an optical carrier with a wavelength of λ2, which, together with the demodulated signal V1, is input into electro-optical intensity modulator 2, which modulates the demodulated signal V1 onto the λ2 optical carrier and inputs optical circulator 2, which specifies the transmission path of the optical signal and transmits it back to the near end.

[0012] Optical circulator 1 receives the return signal, specifies the transmission path of the optical signal, and transmits it to the remote end. Optical circulator 2 receives the optical signal, passes through the wavelength division multiplexer, and outputs the optical carrier with a wavelength of λ2 from the lower branch. After being demodulated by photodetector 2, the RF signal V3 is output. Together with the demodulated signal V2, it is input into the conjugate mixer and passed through bandpass filter 2 to filter out the phase-stable signal V4 of the specified frequency.

[0013] The near end also includes a directional coupler and an erbium-doped fiber amplifier. The modulated signal from the electro-optical intensity modulator 1 is input into the optical circulator 1 through the directional coupler. The optical circulator 1 receives the return signal, specifies the transmission path of the optical signal, and does not transmit it to the far end. The return signal is amplified by the erbium-doped fiber amplifier and input into the directional coupler together with the modulated signal. The modulated signal of the λ1 optical carrier and the return signal of the λ2 optical carrier are coupled into one path, which is input into the optical circulator 1, specifies the transmission path of the optical signal, and is transmitted to the far end.

[0014] Assuming that the angular frequency of the signal to be transmitted is ω0, the center frequency of the bandpass filter 1 is set to ω0, the center frequency of the bandpass filter 2 is set to 2*ω0, and the center frequency of the bandpass filter 3 is set to 3*ω0.

[0015] There are n remote ends with the same architecture. The near end also includes an optical beam splitter. The optical signal output by optical circulator 1 passes through the optical beam splitter, which divides the optical signal into n paths with equal power and transmits them to n remote ends. The continuous wave lasers of the second to n paths generate optical carriers with wavelengths of λ3 to λn+1. The center wavelength of the upper branch passing through the wavelength division multiplexer is λ1, and the center wavelength of the lower branch is λ3 to λn+1. The return signal passes through the optical beam splitter and is transmitted to optical circulator 1. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a block diagram of the single transmission principle. Figure 2 This is the multi-transmission principle block diagram. Figure 3 is the simulated spectrum diagram, Figure 4 This is a comparison diagram of phase changes with disturbance. Implementation Method

[0017] The technical solution of the present invention is described in detail below with reference to the accompanying drawings.

[0018] Assuming a steady-state transmission from a near end to a far end, the principle block diagram is as follows Figure 1 As shown, a continuous wave laser 1 generates an optical carrier with a wavelength of λ1, which, together with the signal V0 to be transmitted, is input into an electro-optical intensity modulator 2. The signal V0 to be transmitted is modulated onto the λ1 optical carrier. The signal passes through a directional coupler 3 and is input into an optical circulator 4. The signal is then transmitted through an optical fiber to an optical circulator 2 7.

[0019] Optical circulator 2 7 receives the optical signal and specifies the transmission path of the optical signal. After passing through wavelength division multiplexer 8, the optical carrier with wavelength λ1 is output from the upper branch. The RF signal is demodulated by photodetector 1 9 and input into electrical power splitter 11, which splits the demodulated signal into two paths with equal power. The demodulated signals are respectively filtered through bandpass filter 12 and bandpass filter 3 13 to obtain demodulated signals V1 and V2 of the specified frequency.

[0020] The continuous wave laser 14 generates an optical carrier with a wavelength of λ2, which, together with the demodulated signal V1, is input into the electro-optical intensity modulator 15. The demodulated signal V1 is modulated onto the λ2 optical carrier and fed back to the optical circulator 7. The optical fiber is then transmitted back to the optical circulator 4.

[0021] Optical circulator 1 4 receives the return signal, passes it through an erbium-doped fiber amplifier 5, amplifies the power of the optical signal, and then inputs it into a directional coupler 3. The modulated signal of the λ1 optical carrier and the return signal of the λ2 optical carrier are coupled into one path, which is then input into optical circulator 1 4 and transmitted again through the optical fiber to optical circulator 2 7.

[0022] The optical signal passes through the wavelength division multiplexer 8, and the optical carrier with a wavelength of λ2 is output from the upper branch. The radio frequency signal V3 is demodulated by the photodetector 10, and together with the demodulated signal V2, it is input into the conjugate mixer 16. After passing through the bandpass filter 17, the mixing signal V4 of the specified frequency is filtered out and can be used as the final stable phase signal.

[0023] Electro-optical intensity modulator 2 uses double-sideband modulation and operates at a quadrature bias point. The power of the transmitted signal V0 exceeds the 1dB power compression point of electro-optical modulator 2, generating a significant third-order nonlinear signal. Photodetector 9 demodulates the RF signal from the λ1 optical carrier, which contains a first-order signal component and a third-order nonlinear component. The demodulated signal V1 filtered by bandpass filter 12 is the first-order signal component, and the demodulated signal V2 filtered by bandpass filter 3 13 is the third-order signal component.

[0024] The demodulated signal V1 is modulated onto the λ2 optical carrier and passes through optical circulator 2 7 and optical circulator 1 4, where the signal strength weakens. The signal is then input into an erbium-doped fiber amplifier 5 for amplification. The signal then passes through a directional coupler 3 and into optical circulator 1 for further transmission. It is then separated from the λ1 optical carrier by a wavelength division multiplexer 8 and input into photodetector 2 10 to demodulate the RF signal V3, which is obtained after V1 has been transmitted back and forth through the optical fiber once, and has the same delay disturbance as V1.

[0025] The demodulated signal V3 and the demodulated signal V2 are input into the conjugate mixer 16, and the mixed signal V4 with a center frequency of 2*ω0 is filtered out by the bandpass filter 17. It does not contain the fiber delay disturbance term ΔT and can be used as a steady-phase signal. The frequency is 2*ω0, and the original signal can be obtained through the divider.

[0026] Assume that φ0 is the initial phase of the signal to be transmitted, T is the delay of the signal from the near end to the far end, which is determined by the intrinsic length of the optical fiber, and ΔT is the optical fiber delay disturbance caused by temperature and stress vibration. Then the signals V0 to V4 can be expressed by the formula express.

[0027] The above scheme simultaneously utilizes the first-order and third-order nonlinear components of the transmission signal to eliminate the additional delay disturbance of the link. The phase-stabilizing system is located at the remote end, and a single-stage mixing achieves phase stability, which simplifies the system structure and outputs the second frequency of the transmission signal. The phase does not change with the fiber disturbance during the transmission process, reducing the deterioration of signal noise in subsequent frequency-doubling links and reducing the loss, noise and spurious introduced by multi-stage mixing.

[0028] VPI Photonics and Optisystem optical link simulation software were used for analysis. Because signal transmission in a closed loop cannot be simulated, the signal was transmitted back and forth three times through the long optical fiber between the near and far ends. This is equivalent to three sections of long optical fiber with identical parameters. The delay disturbance is equivalent to three delay disturbance terms with identical parameters after the three sections of long optical fiber.

[0029] The wavelength of continuous wave laser 1 is set to 1554.4nm (193THz), and the signal to be transmitted V0 is set to 10GHz. It is sent to photodetector 9 via a 10km long optical fiber, and the 10GHz signal V1 and the 30GHz signal V2 are filtered out respectively. The wavelength of continuous wave laser 14 is set to 1552.8nm (193.2THz), loaded with V1, and transmitted back to optical circulator 4 via a 10km long optical fiber with the same parameters. It is sent to photodetector 2 10 via the 10km long optical fiber to obtain V3. V2 and V3 are mixed in mixer 16 and filtered by bandpass filter 2 17 to obtain 20GHz signal V4.

[0030] The simulated spectrum from V0 to V4 is as follows Figure 3As shown, the delay of the 20GHz signal to produce a 2π phase change is 50ps. After changing the three sections of long optical fiber, the delay disturbance ΔT is 0ps, 16.6ps and 33.3ps respectively. The time domain sine spectrum of V4 is as follows Figure 4 As shown in the upper, middle and lower figures on the left, the positions of the peaks and troughs do not change, indicating that the phase of V4 does not change with the change of the delay disturbance ΔT, and the stable phase output of the signal is achieved. Compared with the unstable phase link, Figure 4 As shown in the upper, middle, and lower figures on the right, after the signal is transmitted through a long optical fiber, the first-order signal and the third-order nonlinear signal are recovered. These two signals are mixed to generate a 20 GHz signal. This changes the delay perturbation ΔT in this link to 0 ps, 16.6 ps, and 33.3 ps, respectively. The peak and trough positions are shifted, and the amount of delay shift is the same as the change in the delay perturbation ΔT.

[0031] In order to realize distributed transmission, an optical beam splitter 6 is added to the output end of the optical circulator 4, and n long optical fibers are connected to transmit signals to n remote ends, such as Figure 2 As shown, the wavelength of the optical carrier generated by the second continuous wave laser is λ3, the center wavelength of the upper branch of the wavelength division multiplexer is λ1, and the center wavelength of the lower branch is λ3. The wavelength of the optical carrier generated by the third continuous wave laser is λ4, the center wavelength of the upper branch of the wavelength division multiplexer is λ1, and the center wavelength of the lower branch is λ4. The wavelength of the optical carrier generated by the nth continuous wave laser is λn+1, the center wavelength of the upper branch of the wavelength division multiplexer is λ1, and the center wavelength of the lower branch is λn+1. Theoretically, there is no limit.

[0032] The above are embodiments of the present invention and do not limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention are included in the protection scope of the present invention.

Claims

1. A passive phase-stabilized radio frequency optical transmission system, comprising a proximal end and a distal end, characterized in that: The near-end system includes a continuous wave laser, an electro-optical intensity modulator and an optical circulator, and the far-end system includes an electro-optical intensity modulator, a continuous wave laser, an optical circulator, a wavelength division multiplexer, two photodetectors, an electric power splitter, three band-pass filters and a conjugate mixer; the continuous wave laser generates an optical carrier with a wavelength of λ1, which is input into the electro-optical intensity modulator together with the signal to be transmitted V0, modulated onto the λ1 optical carrier, and input into the optical circulator to specify the transmission path of the optical signal and transmit it to the far-end; the optical circulator receives the optical signal, passes through the wavelength division multiplexer, and outputs the optical carrier with a wavelength of λ1 from the upper branch, demodulates the radio frequency signal through the photodetector, and inputs the electric power splitter to divide the demodulated signal into two equal powers. The optical signal is transmitted through the optical circulator 2, which receives the optical signal and transmits it to the far end. The optical circulator 1 receives the return signal and transmits it to the far end. The optical circulator 2 receives the optical signal and transmits it to the far end. The optical signal is received by the optical circulator 2, which receives the optical signal and transmits it to the far end. The optical signal is transmitted through the wavelength division multiplexer, and the optical signal with a wavelength of λ2 is output from the lower branch. The optical signal is demodulated by the photodetector 2 to obtain the RF signal V3, which is input into the conjugate mixer together with the demodulated signal V2. The signal passes through the bandpass filter 2 and filters out the phase-stable signal V4 of the specified frequency. The proximal end further includes a directional coupler and an erbium-doped fiber amplifier. The modulated signal of the electro-optical intensity modulator 1 is input into the optical circulator 1 through the directional coupler. The optical circulator 1 receives the return signal, specifies the transmission path of the optical signal, and does not transmit it to the far end. The return signal is amplified by the erbium-doped fiber amplifier, and is input into the directional coupler together with the modulated signal. The modulated signal of the λ1 optical carrier and the return signal of the λ2 optical carrier are coupled into one path, which is input into the optical circulator 1, specifies the transmission path of the optical signal, and is transmitted to the far end. There are n remote ends with the same architecture. The near end also includes an optical beam splitter. The optical signal output by the first optical circulator passes through the optical beam splitter, and the optical signal is divided into n paths with equal power, and transmitted to the n remote ends. The continuous wave lasers of the second to n paths generate optical carriers with wavelengths of λ3 to λn+1, the center wavelength of the upper branch passing through the wavelength division multiplexer is λ1, and the center wavelength of the lower branch passing through the wavelength division multiplexer is λ3 to λn+1. The return signal passes through the optical beam splitter and is transmitted to the first optical circulator. Electro-optical intensity modulator 1 uses double-sideband modulation and operates at a quadrature bias point. The power of the to-be-transmitted signal V0 exceeds the 1dB power compression point of electro-optical modulator 1, generating a significant third-order nonlinear signal. Photodetector 1 demodulates the RF signal from the λ1 optical carrier, which contains a first-order signal component and a third-order nonlinear component. The demodulated signal V1 filtered out by bandpass filter 1 is a first-order signal component, and the demodulated signal V2 filtered out by bandpass filter 3 13 is a third-order signal component.

2. The passive phase-stabilized radio frequency optical transmission system according to claim 1, characterized in that: It also includes: assuming that the angular frequency of the signal to be transmitted is ω0, the center frequency of the bandpass filter 1 is set to ω0, the center frequency of the bandpass filter 2 is set to 2*ω0, and the center frequency of the bandpass filter 3 is set to 3*ω0.

3. The passive phase-stabilized radio frequency optical transmission system according to claim 2, characterized in that: Also includes: φ 0 is the initial phase of the signal to be transmitted, T is the delay of the signal from the near end to the far end, which is determined by the intrinsic length of the optical fiber, and ΔT is the optical fiber delay disturbance caused by temperature and stress vibration. The signals V0 to V4 can be expressed by the formula: 。

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