A laser homodyne coherent detection device based on an oscillation optical phase-locked loop

Abstract

The application provides a laser homodyne coherent detection device based on an oscillation optical phase-locked loop, a modulated signal to be transmitted is loaded on an oscillation signal intensity modulated optical carrier as signal laser through phase modulation, and the signal laser is optically mixed with local laser, amplified and input into a balanced detector; the output of the balanced detector is divided into two paths, one path is used for signal recovery, and the other path is input into an optical phase-locked loop, mixed with an oscillation signal, then passed through a low-pass filter to obtain a phase error signal, then passed through a loop filter and an integrator to obtain a phase control signal, and then input into a phase modulator 2 of the local laser. When the system is in a closed loop state, the phases of the local laser and the signal laser are continuously close to each other, and finally phase-locked. The application can realize demodulation of digital signals and analog signals, avoids the phenomenon of further reduction of signal-to-noise ratio caused by power detection and squaring operation on low signal-to-noise ratio signals, and can realize high-sensitivity coherent detection.

Description

Technical Field

[0001] This invention belongs to the field of laser coherent detection devices in the field of optoelectronic technology, specifically relating to a laser zero-difference coherent detection device based on an oscillating optical phase-locked loop. This device can be applied to fields such as high-precision optical sensing, coherent optical communication, and coherent lidar. Background Technology

[0002] Direct detection, as a traditional photoelectric detection method, is limited by its own detection principle, resulting in significant noise mixing in the signal, a low signal-to-noise ratio, and difficulty in receiving laser phase information. Coherent detection technology, on the other hand, boasts advantages such as high detection sensitivity, strong anti-interference capability, high bandwidth utilization, and the ability to demodulate laser phase information, making it a promising field with significant research value and broad application prospects in lidar, high-precision optical sensing, and other areas.

[0003] F. Herzog et al. from ETH Zurich designed a novel optical phase-locked loop (PLL) called an oscillator loop for use in zero-difference inter-satellite optical communication (Herzog F, Kudielka K, Erni D, et al. Optical phase locking by local oscillator phase dithering[J]. IEEE journal of quantum electronics, 2006, 42(10): 973-985.). The device mainly consists of a local oscillator laser, a signal laser, a 180° optical mixer, a balanced detector, a transimpedance amplifier, an RF amplifier, a power detector, a bandpass filter, an electrical mixer, a loop filter, an oscillator, and a 90° phase shifter. In this device, the local oscillator laser and the signal laser are mixed in the 180° optical mixer and then input to the balanced detector, followed by the transimpedance amplifier and then the RF amplifier. The RF amplifier and the power detector together form an automatic gain control module, which adjusts the amplitude of the electrical signal output from the previous stage and inputs the adjusted signal to the bandpass filter, and then to the electrical mixer. A small phase perturbation signal (called the oscillation signal) is generated by an oscillator and input to the piezoelectric frequency tuning port of the local oscillator laser and a 90° phase shifter. The output of the 90° phase shifter is connected to another input port of an electric mixer. The signal obtained after the output of the electric mixer is passed through a low-pass filter is proportional to the phase error and can be used for phase control of the local oscillator laser. In phase-locked loop (PLL) mode, the digital phase-modulated signal is directly recovered.

[0004] According to the theoretical analysis by F. Herzog et al., the phase component of the intermediate frequency signal output by the balanced detector in this scheme includes the modulation signal, the oscillation signal, and the phase error signal. For digital modulation signals, the digital modulation signal can be eliminated after power detection and squaring. The output signal of the power detector is then mixed with the oscillation signal by an electric mixer, and the oscillation signal can be eliminated after passing through a filter. Finally, the phase error signal is obtained for phase locking. However, for analog modulation signals, the power detector cannot eliminate the analog modulation signal, and the device cannot demodulate the phase error signal for phase locking. Therefore, this scheme cannot demodulate analog modulation signals, limiting its application range. Secondly, when the optical signal is very weak, resulting in a low signal-to-noise ratio (SNR), squaring the signal by the power detector will further reduce the SNR, thereby reducing the coherent detection sensitivity. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the background technology by proposing a laser homodyne coherent detection device based on an oscillating optical phase-locked loop. This coherent detection device can demodulate digital or analog signals while avoiding the influence of power detectors on the signal-to-noise ratio, thus achieving high-sensitivity coherent detection.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A laser zero-difference coherent detection device based on an oscillating optical phase-locked loop includes a laser, a 50 / 50 fiber coupler 1, an intensity modulator, a phase modulator 1, a phase modulator 2, an oscillator, a 50 / 50 fiber coupler 2, a balanced detector, an electrical mixer, a low-pass filter, a loop filter, and an integrator.

[0008] The laser serves as the source for both the local oscillator laser and the signal laser. Its output beam is split by a 50 / 50 fiber coupler 1 to form two beams of the same frequency. One beam is used as the signal laser and input to the intensity modulator (path 1), while the other beam is used as the local oscillator laser and input to the phase modulator 2 (path 2).

[0009] The oscillator generates an oscillation signal, which is input to the modulation port of the intensity modulator and the input port of the electric mixer, respectively.

[0010] The signal laser input to the intensity modulator is intensity modulated by the oscillation signal and then output to the phase modulator 1, where it is phase modulated by the modulation signal before being input to the 50 / 50 fiber coupler 2. The local oscillator laser input to the phase modulator 2 is phase modulated by the phase error signal from the phase-locked loop and then input to the 50 / 50 fiber coupler 2. The two outputs of the 50 / 50 fiber coupler 2 are connected to the two inputs of the balanced detector. The two laser beams input to the 50 / 50 fiber coupler 2 are mixed in the 50 / 50 fiber coupler 2 and then received by the balanced detector. The balanced detector then converts the mixed light into a frequency-modulated signal. After the frequency signal is converted into an electrical signal, two intermediate frequency signals are output. One signal is used for signal recovery, and the other is input to an electric mixer to mix with the oscillation signal. The output of the electric mixer is connected to the input of a low-pass filter, which filters out the oscillation signal and retains only the phase error signal. The output of the low-pass filter is connected to the input of a loop filter, which filters out high-frequency noise and spurious components. The output of the loop filter is connected to the input of an integrator, which integrates the signal to obtain the phase control signal. The output of the integrator is connected to the modulation port of phase modulator 2, which changes the phase of the local oscillator light to achieve phase locking.

[0011] Furthermore, the laser is a distributed feedback semiconductor laser (DFB).

[0012] Furthermore, the modulation signal input to the phase modulator 1 can be an analog signal or a digital signal.

[0013] This invention provides a laser coherent detection device based on an oscillating optical phase-locked loop. The modulated signal to be transmitted is loaded onto an optical carrier modulated by the intensity of the oscillating signal as a signal laser. This signal laser is optically mixed with the local oscillator laser and amplified before being input to a balanced detector. The output of the balanced detector is split into two paths: one for signal recovery and the other input to the optical phase-locked loop. After mixing with the oscillating signal, the signal is passed through a low-pass filter to obtain a phase error signal. This signal is then passed through a loop filter and an integrator to obtain a phase control signal, which is subsequently input to the phase modulator 2 of the local oscillator laser. When the system is in a closed-loop state, the phases of the local oscillator laser and the signal laser will continuously approach each other, eventually achieving phase lock. In the phase-locked state, the balanced detector can directly output the modulated signal to achieve signal recovery. This invention can demodulate both digital and analog signals and avoids the further reduction in signal-to-noise ratio caused by power detection and squaring operations on low signal-to-noise ratio signals, enabling high-sensitivity coherent detection.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] 1. This invention provides a laser homodyne coherent detection device based on an oscillating optical phase-locked loop. The oscillating signal and the modulation signal are loaded onto the signal light through intensity modulation and phase modulation, respectively. The phase portion of the intermediate frequency signal output by the balanced detector contains the modulation signal and the phase error signal. The phase error signal can be obtained by direct demodulation for phase locking. After phase locking is completed, the modulation signal can be recovered. This scheme is applicable to both digital and analog signals.

[0016] 2. This invention does not use a power detector to perform squaring operations on the intermediate frequency signal, thus avoiding the phenomenon of further reduction in signal-to-noise ratio caused by power detection and squaring operations on low signal-to-noise ratio signals, and can achieve highly sensitive coherent detection. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a laser zero-difference coherent detection device based on an oscillating optical phase-locked loop, provided by the present invention. Detailed Implementation

[0018] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0019] Example

[0020] like Figure 1 The diagram shown is a schematic of a laser zero-difference coherent detection device based on an oscillating optical phase-locked loop provided by the present invention, including a laser, a 50 / 50 fiber coupler 1, an intensity modulator, a phase modulator 1, a phase modulator 2, an oscillator, a 50 / 50 fiber coupler 2, a balanced detector, an electric mixer, a low-pass filter, a loop filter, and an integrator.

[0021] The laser beam output from the laser is split into a signal laser (path 1) and a local oscillator laser (path 2) by a 50 / 50 fiber coupler 1. The signal laser is input to phase modulator 1 after passing through an intensity modulator, and then serves as one input to the 50 / 50 fiber coupler 2. The modulation signal of the intensity modulator comes from the oscillator, while the modulation signal of phase modulator 1 can be a sensor signal acquired from the sensing system or a data signal from the coherent optical communication system. The local oscillator laser is input to the 50 / 50 fiber coupler 2 after passing through phase modulator 2, where the modulation signal of phase modulator 2 comes from the oscillation phase-locked loop. The phase control signal output by the integrator; the modulated local oscillator laser and the signal laser are optically mixed in a 50 / 50 fiber coupler 2 and then input to a balanced detector to be converted into an electrical signal. The output of the balanced detector is split into two paths, one for signal recovery and the other for input to an electrical mixer, where it is mixed with the oscillation signal generated by the oscillator. Then, the phase error signal is obtained through a low-pass filter and input to a loop filter. After being connected to the integrator, it is input to the phase modulator 2 to complete the closed loop. During the phase-locking process, the phase difference between the local oscillator laser and the signal laser gradually approaches zero, and finally the phase lock between the local oscillator laser and the signal laser is completed.

[0022] This invention utilizes a laser coherent detection method with zero difference to achieve phase locking between the local oscillator laser and the signal laser through an oscillating optical phase-locked loop. The oscillation signal generated by the oscillator is loaded into the optical phase-locked loop. When the loop is not locked, the phase error signal can be demodulated by a balanced detector and an electrical mixer. The signal, after filtering by the loop filter and processing by the integrator, can be used as a phase control signal (the modulation signal of phase modulator 2) to adjust the phase of the local oscillator laser, gradually reducing the phase difference between the local oscillator laser and the signal laser to achieve loop locking. In the phase-locked state, the balanced detector can directly output the modulation signal, thereby achieving signal recovery.

Claims

1. A laser homodyne coherent detection device based on an oscillating optical phase-locked loop, characterized in that, It includes a laser, a 50 / 50 fiber coupler 1, an intensity modulator, a phase modulator 1, a phase modulator 2, an oscillator, a 50 / 50 fiber coupler 2, a balanced detector, an electric mixer, a low-pass filter, a loop filter, and an integrator. The laser serves as the source for both the local oscillator laser and the signal laser. Its output beam is split by a 50 / 50 fiber coupler 1 to form two beams of light with the same frequency. One beam is used as the signal laser and input to the intensity modulator, while the other beam is used as the local oscillator laser and input to the phase modulator 2. The oscillator generates an oscillation signal, which is input to the modulation port of the intensity modulator and the input port of the electric mixer, respectively. The signal laser input to the intensity modulator is intensity modulated by the oscillation signal and then output to the phase modulator 1, where it is phase modulated by the modulation signal before being input to the 50 / 50 fiber coupler 2. The local oscillator laser input to the phase modulator 2 is phase modulated by the phase control signal output from the phase-locked loop before being input to the 50 / 50 fiber coupler 2. The two outputs of the 50 / 50 fiber coupler 2 are connected to the two inputs of the balanced detector. The two laser beams are mixed in the 50 / 50 fiber coupler 2 and then received by the balanced detector, which converts the optical mixing signal into electrical signals. After the signal is received, the output intermediate frequency signal is split into two paths. One path is used for signal recovery, and the other path is input to the electric mixer to mix with the oscillation signal. The output of the electric mixer is connected to the input of the low-pass filter, which filters out the oscillation signal and retains only the phase error signal. The output of the low-pass filter is connected to the input of the loop filter, which filters out high-frequency noise and spurious components. The output of the loop filter is connected to the input of the integrator, which integrates the signal to obtain the phase control signal. The output of the integrator is connected to the modulation port of the phase modulator 2, which changes the phase of the local oscillator light to achieve phase locking.

2. The laser homodyne coherent detection device based on an oscillating optical phase-locked loop according to claim 1, characterized in that, The laser is a distributed feedback semiconductor laser.

3. The laser homodyne coherent detection device based on an oscillating optical phase-locked loop according to claim 1, characterized in that, The modulation signal input to the phase modulator 1 is an analog signal or a digital signal.

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