Homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signal

Abstract

The invention discloses a homodyne coherent detection structure for receiving a single-photon-level broadband optical pulse signal, and belongs to the technical field of photoelectrons. The homodyne coherent detection structure comprises an oscillation signal generator, a signal processing module 1, a signal processing module 2, a band-pass filter 1, a band-pass filter 2, a phase modulator 1, a phase modulator 2, an intensity modulator, a 180-degree optical mixer, a photoelectric detector 1, a photoelectric detector 2, an amplifier 1, an amplifier 2, a power divider 1, a power divider 2, a balun, a mixer and a low-pass filter. A loop filter and an integrator. Phase synchronization of signal light and local oscillation light is guaranteed through the optical phase-locked loop structure, the two interference output branches are independently detected through the two single-port photoelectric detectors, compared with a traditional single-path output structure, the detection signal-to-noise ratio can be increased by about 2dB, and detection and demodulation of single-photon-magnitude bandwidth pulse optical signals are achieved.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic technology, and particularly relates to a homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals, which can be applied to fields such as lidar, laser sensing, and quantum optics. Background Art

[0002] Optoelectronic detection can be divided into direct detection and coherent detection. The conventional optoelectronic detector used in direct detection can generate a photocurrent proportional to the incident optical power. However, limited by factors such as thermal noise and electronic noise of the amplifier circuit, it is difficult to achieve single-photon level detection. The single-photon detector improves the sensitivity of the optoelectronic detector to the quantum limit. Cryogenic single-photon detectors represented by superconducting nanowire single-photon detectors can achieve single-photon detection, and the detection efficiency can reach more than 80%. However, cryogenic single-photon detectors need to work in an environment close to absolute zero, with a large volume, heavy weight, high power consumption, and complex systems, which to a certain extent limits their application scope.

[0003] Coherent detection is divided into homodyne coherent detection and heterodyne coherent detection. Among them, the sensitivity of homodyne coherent detection is 3 dB higher than that of heterodyne coherent detection. Traditional homodyne coherent detection is mainly applied to the field of optical communication, and is mostly used to realize the demodulation of digitally phase-modulated optical signals. There is less research and application on homodyne coherent detection of amplitude-modulated or pulsed optical signals.

[0004] In view of the detection requirements for extremely weak pulsed optical signals in the fields of lidar, laser sensing, and quantum optics, the present invention proposes a homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals, which can work at room temperature and has high detection sensitivity. Summary of the Invention

[0005] The object of the present invention is to propose a homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals in view of the detection requirements for extremely weak pulsed optical signals in the fields of lidar, laser sensing, and quantum optics, and to realize the demodulation of single-photon level broadband pulsed optical signals. The present invention utilizes the phase relationship between single-photon pulses, and by adding a phase oscillation signal, optical phase locking based on an AC-coupled front end can be realized, improving the stability of phase locking, and based on this, the demodulation of high-sensitivity pulsed optical signals is realized.

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

[0007] A homodyne coherent detection structure for receiving single - photon - level broadband optical pulse signals, comprising an oscillation signal generator, signal processing module 1, signal processing module 2, band - pass filter 1, band - pass filter 2, phase modulator 1, phase modulator 2, intensity modulator, 180° optical mixer, photodetector 1, photodetector 2, amplifier 1, amplifier 2, power splitter 1, power splitter 2, balun, mixer, low - pass filter, loop filter and integrator;

[0008] The oscillation signal generator generates an oscillation signal and is connected to phase modulator 1 and the mixer;

[0009] The signal light is input into phase modulator 1, phase - modulated by the oscillation signal, then input into the intensity modulator for intensity modulation. The output end of the intensity modulator is connected to one input end of the 180° optical mixer, and the signal after phase modulation and intensity modulation is output to the 180° optical mixer; The local oscillator light is input into phase modulator 2, phase - modulated and then input into the other input end of the 180° optical mixer; After the two optical signals pass through the 180° optical mixer, a pair of differential signals are output, which are respectively transmitted to photodetector 1 and photodetector 2. After being converted into voltage signals by photodetector 1 and photodetector 2, they are input into amplifier 1 and amplifier 2 for amplification and then distributed into two signals by power splitter 1 and power splitter 2; One output end of power splitter 1 is connected to one input end of the balun, and the other output end is connected to band - pass filter 1. One output end of power splitter 2 is connected to the other input end of the balun, and the other output end is connected to band - pass filter 2. The signals output by power splitter 1 and power splitter 2 are converted into one signal by the balun and then output to the mixer, where they are multiplied by the oscillation signal to obtain a phase error signal. The phase error signal is filtered by the low - pass filter and the loop filter to remove high - frequency components and DC components, and then input into the integrator. In the integrator, integration is performed to obtain a phase error control signal, and this phase error control signal enters phase modulator 2 to adjust the phase of the local oscillator light to ensure the phase synchronization of the two optical signals;

[0010] When the loop enters the locked state, the signals output by power splitter 1 and power splitter 2 are filtered by band - pass filter 1 and band - pass filter 2, and then demodulated in signal processing module 1 and signal processing module 2, and the demodulated signals are output at signal output 1 and signal output 2 ports.

[0011] Further, when the input signal light is a single - photon - level broadband optical pulse signal, demodulated signals will be output at signal output 1 and signal output 2 ports, and the single - photon - level broadband optical pulse signal can be detected by using a spectrum analyzer or an oscilloscope.

[0012] Further, the bandwidth of photodetector 1 and photodetector 2 is 20 GHz, the responsivity is 1.2 A / W, and the quantum efficiency is 96%.

[0013] Further, the bandwidths of the amplifier 1 and the amplifier 2 are 20 GHz, and the gains are 20 dB.

[0014] Further, the bandwidth of the balun is 26.5 GHz.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] The present invention provides a homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals. The phase synchronization between the signal light and the local oscillator light is ensured by an optical phase-locked loop structure. Two single-port photodetectors are used to independently detect two interference output branches. Compared with the traditional single-channel output structure, the detection signal-to-noise ratio can be increased by about 2 dB, and the detection and demodulation of single-photon level bandwidth pulsed optical signals can be realized. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of a homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals provided by the present invention. Detailed Embodiments

[0018] The present invention will be described in detail below with reference to the 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] Embodiment

[0020] A homodyne coherent detection structure for receiving single-photon level broadband optical pulse signals, as Figure 1 shown, includes an oscillation signal generator, a signal processing module 1, a signal processing module 2, a band-pass filter 1, a band-pass filter 2, a phase modulator 1, a phase modulator 2, an intensity modulator, a 180° optical mixer, a photodetector 1, a photodetector 2, an amplifier 1, an amplifier 2, a power splitter 1, a power splitter 2, a balun, a mixer, a low-pass filter, a loop filter, and an integrator;

[0021] The oscillation signal generator generates an oscillation signal and is connected to the phase modulator 1 and the mixer;

[0022] The signal light is input into the phase modulator 1. After being phase-modulated by the oscillation signal, it is input into the intensity modulator for intensity modulation. The output end of the intensity modulator is connected to one input end of the 180° optical mixer. The signal after phase modulation and intensity modulation is output to the 180° optical mixer; the local oscillator light is input into the phase modulator 2, and after being phase-modulated, it is input into the other input end of the 180° optical mixer; after the two optical signals pass through the 180° optical mixer, a pair of differential signals are output, which are respectively transmitted to the photodetector 1 and the photodetector 2. After being converted into voltage signals by the photodetector 1 and the photodetector 2, they are input into the amplifier 1 and the amplifier 2 for amplification, and are divided into two signals by the power splitter 1 and the power splitter 2; one output end of the power splitter 1 is connected to one input end of the balun, and the other output end is connected to the band-pass filter 1. One output end of the power splitter 2 is connected to the other input end of the balun, and the other output end is connected to the band-pass filter 2. The signals output by the power splitter 1 and the power splitter 2 are converted into one signal by the balun and then output to the mixer, where they are multiplied by the oscillation signal to obtain the phase error signal. After the phase error signal filters out the high-frequency components and the DC component through the low-pass filter and the loop filter, it is input into the integrator, and the phase error control signal is obtained by integrating in the integrator. This phase error control signal enters the phase modulator 2 to adjust the phase of the local oscillator light to ensure the phase synchronization of the two optical signals;

[0023] After the loop enters the locked state, the signals output by the power splitter 1 and the power splitter 2 are filtered by the band-pass filter 1 and the band-pass filter 2, and then demodulated in the signal processing module 1 and the signal processing module 2, and the demodulated signals are output at the signal output 1 and signal output 2 ports.

[0024] Assume that the optical field expression of the input signal light is

[0025]

[0026] In the formula, P S , ω S , are respectively the optical power, frequency and phase of the signal light. The optical field expression of the local oscillator light is

[0027]

[0028] In the formula, P LO , ω LO , are respectively the optical power, frequency and phase of the local oscillator light. The expression of the oscillating optical signal is:

[0029] U dither =U d cosω d t(0.3)

[0030] Among them, Ud and ω d are the amplitude and frequency of the oscillation signal respectively. After the signal light passes through the phase modulator 1 and the intensity modulator, we can get:

[0031]

[0032] P(t) is the power of the signal light after modulation, A d is the phase modulation coefficient, Δθ represents the phase difference between the signal light and the local oscillator light. Assuming that there is no phase error signal in the initial state of the system, the local oscillator light after modulation can be obtained as

[0033]

[0034] After passing through the 180° optical mixer, the power expressions of the two optical signals can be obtained as:

[0035]

[0036] After passing through the photodetector, considering the 50-ohm impedance matching between the photodetector and the peripheral circuit, the voltage expression entering the amplifier can be obtained as:

[0037]

[0038] R D is the responsivity of the photodetector, R L is the 50-ohm impedance. After passing through the amplifier, power splitter and band-pass filter, the expression of the output signal is:

[0039]

[0040] In the formula, is the 3dB power loss caused by the power splitter, which is converted to a voltage loss of 0.707. G 1 represents the voltage gain of the amplifier. When the loop is locked, that is, when Δθ→0, using the Bessel function to simplify, the demodulation signal expression can be obtained as

[0041]

[0042] Considering the shot noise and thermal noise generated by the photodetector in this system, the signal-to-noise ratio at the signal output end can be obtained as:

[0043]

[0044] In the formula, Δf represents the spectral bandwidth of the signal light, q is the electronic charge, k is the Boltzmann constant, T is the temperature. When the signal light P(t) is Gaussian pulse light, its expression is

[0045] P(t) = P Sexp[-2(t / τ) 2 (0.11)

[0046] At this time, in Equation (0.10), Δf is the single-sideband bandwidth of the Gaussian pulse signal. The laser pulse width τ defined according to the full width at half maximum p The relationship with τ is Substituting and simplifying gives Δf = 0.221 / τ p . When the number of photons contained in the signal laser pulse is N, there is a relationship Nhν = P S τ p . When the local oscillator optical power is large enough, Equation (0.10) can be simplified to:

[0047]

[0048] Assume the phase modulation coefficient A d is 0.6. When the quantum efficiency η of the photodetector reaches 100%, for single-photon detection, when N = 1, the signal-to-noise ratio can reach 4 dB.

[0049] For the traditional single-channel output structure, the noise power is twice that of this structure, and its signal-to-noise ratio expression is:

[0050]

[0051] Similarly, it can be calculated that its signal-to-noise ratio is about 2 dB. Through principle analysis and calculation, it shows that the detection performance of the structure of the present invention is superior to that of the traditional detection structure.

Claims

1. A homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signals, characterized in that: It includes an oscillation signal generator, a signal processing module 1, a signal processing module 2, a bandpass filter 1, a bandpass filter 2, a phase modulator 1, a phase modulator 2, an intensity modulator, a 180° optical mixer, a photodetector 1, a photodetector 2, an amplifier 1, an amplifier 2, a power divider 1, a power divider 2, a balun, a mixer, a low-pass filter, a loop filter and an integrator; The oscillation signal generator generates an oscillation signal, which is connected to the phase modulator 1 and the mixer; The signal light is input into phase modulator 1, and after phase modulation using the oscillation signal, it is input into intensity modulator for intensity modulation. The output end of the intensity modulator is connected to one input end of the 180° optical mixer, and the signal after phase modulation and intensity modulation is output to the 180° optical mixer; the local oscillator light is input into phase modulator 2, and after phase modulation, it is input into the other input end of the 180° optical mixer; after the two optical signals pass through the 180° optical mixer, a pair of differential signals are output, which are transmitted to photodetector 1 and photodetector 2 respectively, and after being converted into voltage signals by photodetector 1 and photodetector 2, they are input into amplifier 1 and amplifier 2 for amplification, and power divider 1 and power divider 2 are distributed into two paths. signal; one output end of the power divider 1 is connected to one input end of the balun, and the other output end is connected to the bandpass filter 1; one output end of the power divider 2 is connected to the other input end of the balun, and the other output end is connected to the bandpass filter 2; the signals output by the power divider 1 and the power divider 2 are converted into one signal by the balun, and then output to the mixer, and multiplied with the oscillation signal in the mixer to obtain a phase error signal; the phase error signal is filtered out by a low-pass filter and a loop filter to remove high-frequency components and DC components, and then input to the integrator, and integrated in the integrator to obtain a phase error control signal, and the phase error control signal enters the phase modulator 2 to adjust the phase of the local oscillator light to ensure the phase synchronization of the two paths of light; When the loop enters the locked state, the signals output by power divider 1 and power divider 2 are filtered by bandpass filter 1 and bandpass filter 2, demodulated in signal processing module 1 and signal processing module 2, and the demodulated signals are output at signal output 1 and signal output 2 ports.

2. The homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signals according to claim 1, characterized in that: When the input signal light is a single-photon broadband optical pulse signal, a demodulated signal will be output at the signal output 1 and signal output 2 ports, and a single-photon broadband optical pulse signal can be obtained by detection using a spectrum analyzer or an oscilloscope.

3. The homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signals according to claim 1, characterized in that: The bandwidth of the photodetector 1 and the photodetector 2 is 20 GHz, the responsivity is 1.2 A / W, and the quantum efficiency is 96%.

4. The homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signals according to claim 1, characterized in that: The bandwidth of the amplifier 1 and the amplifier 2 is 20 GHz, and the gain is 20 dB.

5. The homodyne coherent detection structure for receiving single-photon-level broadband optical pulse signals according to claim 1, characterized in that: The bandwidth of the balun is 26.5 GHz.

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