Laser phase noise measurement method and device

Abstract

The invention discloses a laser phase noise measuring method. The method comprises the following steps: step 1, dividing an output optical signal of a laser into two paths, taking one path as signal light after time delay tau is introduced, and taking the other path as local oscillation light; step 2, mixing the signal light and the local oscillation light by using a 90-degree optical mixer, and selecting two paths of output optical signals with orthogonal phases from four paths of output optical signals of the 90-degree optical mixer to respectively perform photoelectric detection to obtain two paths of electric signals; 3, respectively carrying out analog-to-digital conversion on the two paths of electric signals to obtain two paths of corresponding digital signals VI (t) and VQ (t); and step 4, obtaining the phase noise L (fm) of the laser through digital signal processing, wherein # imgabs0 # S psi (fm) represents the power spectrum density of the digital signal # imgabs1 #. The invention further discloses a laser phase noise measuring device. Compared with the prior art, the measuring precision is higher, the system structure is simpler, and the measurement is faster and more convenient.

Description

Technical Field

[0001] The invention relates to the technical field of microwave photon measurement, and in particular to a method and device for measuring laser phase noise. Background Art

[0002] Light source is an indispensable key component in optical communication and fiber optic sensing systems. As fiber optic communication develops towards higher frequencies, the requirements for light sources are getting higher and higher. As a source of coherent light, lasers play a very important role in optical communication, microwave photonic systems, quantum information processing and other fields. In communication systems, the channel capacity of signal transmission is determined by the channel bandwidth and the signal-to-noise ratio of the system. When the channel bandwidth is constant, the lower the system noise, the higher the signal-to-noise ratio, and the greater the channel capacity. Therefore, under the premise that there is limited room for channel bandwidth improvement, the current high-speed and high-capacity communication system requires the light source to have low noise characteristics. Under ideal conditions, the output light of the laser is all generated by stimulated radiation, and its phase and amplitude are constant. However, the actual laser output will be affected by random spontaneous radiation, and the phase and intensity of the output light will be disturbed, generating noise. The presence of intensity noise reduces the signal-to-noise ratio of the system, reduces the sensitivity of the receiver, and affects the performance of the entire microwave optical link. Phase noise will cause a series of problems such as increased bit error rate and spectral line broadening in the application system. However, as the requirements for the line width and noise of laser light sources in the fields of optical communications, satellite communications, and lidar become higher and higher, new challenges have been raised for the measurement technology of laser noise, and new high-precision measurement methods are urgently needed to measure the noise characteristics of lasers. Accurately measuring laser noise can not only be used as an evaluation indicator of lasers to provide support and guidance for their application range, but also the measurement value can be used to evaluate the specific link performance of the laser application, and then the link power compensation or noise suppression can be performed to improve the link performance.

[0003] At present, there are many methods for testing laser phase noise, and the commonly used ones are phase detection method, frequency detection method and cyclic self-heterodyne method. Among them, the phase detection method and frequency detection method can suppress amplitude noise to a certain extent and realize the accurate separation of phase noise and amplitude noise. However, the phase detection method has high requirements for the line width and phase noise of the reference laser, while the frequency detection method does not require a reference laser, which reduces the measurement requirements. However, the calibration of the frequency detection method is complicated, and the calibration results will also be affected by amplitude noise. Both the phase detection method and the frequency detection method require that the delay caused by the optical fiber must be much greater than the coherence time of the laser to ensure the correctness of the measurement results. The cyclic self-heterodyne method requires precise cyclic delay lines and complex optical devices. The experimental system is relatively complicated to build, the data processing time is long, and the experimental cost is high. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a laser phase noise measurement method, which combines the time-delay self-homodyne technology and the frequency discrimination technology to achieve higher measurement accuracy, simpler system structure, and faster and more convenient measurement.

[0005] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0006] A method for measuring laser phase noise comprises the following steps:

[0007] Step 1, the output optical signal of the laser is divided into two paths, one path is used as signal light after introducing a delay τ, and the other path is used as local oscillator light;

[0008] Step 2, mixing the signal light and the local oscillator light with a 90° optical mixer, and selecting two output optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection, respectively, to obtain two electrical signals;

[0009] Step 3: Perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), V Q (t);

[0010] Step 4: By digital signal processing, the phase noise L(f m ):

[0011]

[0012] S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

[0013] Based on the same inventive concept, the following technical solutions can also be obtained:

[0014] A laser phase noise measurement device, comprising:

[0015] The branching and delay module is used to divide the output optical signal of the laser into two paths, one of which is used as signal light after introducing a delay of τ, and the other is used as local oscillator light;

[0016] The mixing and photoelectric detection module is used to mix the signal light and the local oscillator light with a 90° optical mixer, and select two output optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection to obtain two electrical signals;

[0017] The analog-to-digital conversion module is used to perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), VQ (t);

[0018] A digital signal processing module is used to obtain the phase noise L(f m ):

[0019]

[0020] S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

[0021] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0022] The technical solution of the present invention combines the time-delay self-zero difference technology and the frequency discrimination technology, which can realize the direct measurement of the phase information of the laser output optical signal, and then calculate the phase noise of the laser; compared with the existing technology, the present invention effectively improves the measurement accuracy of the system, and can overcome the shortcomings of the complex structure brought by the phase shifter and feedback loop in the frequency discrimination method and the influence of the phase noise measurement at low frequency offset, making the measurement and analysis of the laser phase noise more convenient. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the structural principle of the laser phase noise measurement device of the present invention. DETAILED DESCRIPTION

[0024] In view of the shortcomings of the prior art, the solution of the present invention is to combine the time-delay self-homodyne technology with the frequency discrimination technology.

[0025] The technical solution proposed by the present invention is as follows:

[0026] A method for measuring laser phase noise comprises the following steps:

[0027] Step 1, the output optical signal of the laser is divided into two paths, one path is used as signal light after introducing a delay τ, and the other path is used as local oscillator light;

[0028] Step 2, mixing the signal light and the local oscillator light with a 90° optical mixer, and selecting two optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection, respectively, to obtain two electrical signals;

[0029] Step 3: Perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), V Q (t);

[0030] Step 4: By digital signal processing, the phase noise L(f m ):

[0031]

[0032] S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

[0033] A laser phase noise measurement device, comprising:

[0034] The branching and delay module is used to divide the output optical signal of the laser into two paths, one of which is used as signal light after introducing a delay of τ, and the other is used as local oscillator light;

[0035] The mixing and photoelectric detection module is used to mix the signal light and the local oscillator light with a 90° optical mixer, and select two output optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection to obtain two electrical signals;

[0036] The analog-to-digital conversion module is used to perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), V Q (t);

[0037] A digital signal processing module is used to obtain the phase noise L(f m ):

[0038]

[0039] S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

[0040] In order to facilitate public understanding, the technical solution of the present invention is described in detail below through a specific embodiment and in conjunction with the accompanying drawings:

[0041] The laser phase noise measurement device in this embodiment is as follows: Figure 1 As shown, it includes a power divider, a 90° optical mixer, a photodetector, an analog-to-digital converter, and a digital signal processing module.

[0042] The working principle of the laser phase noise measurement device is as follows Figure 1 As shown, the details are as follows:

[0043] The output light field of the laser can be expressed as:

[0044]

[0045] Where E is the amplitude of the signal, ω0 is the central angular frequency of the signal, is the phase jitter after the signal is delayed.

[0046] The power divider divides the output optical signal of the laser into two paths. One path passes through the delay optical fiber, generates a delay τ, and then enters the 90° optical mixer as signal light. The signal light can be expressed as:

[0047]

[0048] Among them, E SI is the amplitude of the signal, is the phase jitter after the signal is delayed.

[0049] The other path directly enters the 90° optical mixer as the local oscillator light, and the local oscillator light can be expressed as:

[0050]

[0051] Among them, E LO is the amplitude of the signal.

[0052] The two signals are mixed by the 90° optical mixer and then output into four channels, which are: E SI +E LO 、E SI -E LO 、E SI +jE LO 、E SI -jE LO , the expression is as follows:

[0053]

[0054] Among them, the relative phase differences of E1, E2, E3, and E4 are: 180°, 0°, 270°, and 90°.

[0055] Two orthogonal output signals with a phase difference of 90° (taking E1 and E3 as an example in this embodiment) are input into photodetectors 1 and 2 respectively, and the output voltage signals are:

[0056]

[0057] Among them, R, Z L are the responsivity and input impedance of the photodetector, respectively.

[0058] The output signals of detector 1 and detector 2 are collected by analog-to-digital converters. The DC signals are isolated, and two corresponding digital signals V are obtained. I 、V Q :

[0059]

[0060] Then digital signal processing is performed:

[0061] make but:

[0062]

[0063] Considering that ψ(t) is given by and ω0τ, and ω0 and τ are both constants, so at non-zero frequency, ψ(t) and have the same power spectral density;

[0064] make Then the Fourier transform of θ is From the Fourier transform phase shift theorem, we know that:

[0065]

[0066] where φ(f) is Fourier transform of ; so:

[0067]

[0068] in, for The power spectral density of

[0069] The phase noise of the laser can be obtained:

[0070]

[0071] Among them, L(f m ) is the phase noise of the laser, S ψ (f m ) indicates a digital signal Power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

Claims

1. A method for measuring laser phase noise, characterized in that: The following steps are involved: Step 1, the output optical signal of the laser is divided into two paths, one path is used as signal light after introducing a delay τ, and the other path is used as local oscillator light; Step 2, mixing the signal light and the local oscillator light with a 90° optical mixer, and selecting two output optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection, respectively, to obtain two electrical signals; Step 3: Perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), V Q (t); Step 4: By digital signal processing, the phase noise L(f m ): S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

2. A laser phase noise measurement device, characterized in that: include: The branching and delay module is used to divide the output optical signal of the laser into two paths, one of which is used as signal light after introducing a delay of τ, and the other is used as local oscillator light; The mixing and photoelectric detection module is used to mix the signal light and the local oscillator light with a 90° optical mixer, and select two output optical signals with orthogonal phases from the four output optical signals of the 90° optical mixer for photoelectric detection to obtain two electrical signals; The analog-to-digital conversion module is used to perform analog-to-digital conversion on the two electrical signals to obtain the corresponding two digital signals V I (t), V Q (t); A digital signal processing module is used to obtain the phase noise L(f m ): S ψ (f m ) indicates a digital signal The power spectral density, f m Indicates the frequency deviation relative to the center frequency of the laser.

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