Acousto-optic device turn-off time testing device

Abstract

The utility model provides an acousto-optic device turn-off time testing device which comprises an acousto-optic device connected with a signal source and a laser, continuous laser emitted by the laser is divided into two light beams through a beam splitter, the two light beams face a coupler, and the acousto-optic device is arranged on a light path of one light beam; an optical attenuator is connected behind the acousto-optic device; and the coupler is sequentially connected with the balance detector, the signal acquisition system and the signal processing system. According to the utility model, a pulsed light trailing signal can be amplified, a trailing signal with pW-level power can be detected, the detection sensitivity is effectively improved, and more accurate test of acousto-optic turn-off time is realized.

Description

Technical Field

[0001] This utility model relates to the field of lidar, specifically to a device for testing the off-time of acousto-optic devices. Background Technology

[0002] Wind-measuring lidar is based on coherent detection. It emits pulsed laser light into the atmosphere, which is then scattered by aerosol surfaces. Based on the flight time of the pulsed laser signal and the Doppler frequency shift information it carries, wind speed information at different distances is measured. Most lasers in wind-measuring lidar systems are based on a master oscillator power amplifier structure. A seed laser emits continuous laser light, which is then chopped and frequency-shifted by an acousto-optic device, becoming a pulsed laser light. This pulsed laser light is then amplified by an fiber optic amplifier and emitted into the air through a telescope. Backscattered signals from aerosols at different distances are received by the telescope and then beat with the local oscillator light to demodulate the wind speed information. Because surface reflections in the optical path are unavoidable, the reflected pulsed light beats with the local oscillator light, creating abnormal peaks in the noise spectrum that can overwhelm signals at close range. Therefore, a blind zone exists during detection. Theoretically, the blind zone distance is c × τ / 2, where c is the speed of light and τ is the pulse width of the laser light. The chopping effect of the acousto-optic device essentially applies a switch to the continuous laser light. When the light signal is turned off, there is a turn-off time during which the light signal gradually weakens. Because coherent detection amplifies weak signals using local oscillator light, it exhibits high sensitivity. However, when the reflected light from the internal surface of a wind-measuring lidar system beats with the local oscillator light, the optical signal during the off-time of the acousto-optic devices will generate noise due to this beat. Even when the optical power is reduced to the pW level, abnormal peaks will still occur, leading to an increase in the actual detection blind zone of the system. Currently, the detection blind zone of conventional wind-measuring lidars is around 40 m. To reduce the detection blind zone to 20 m, in addition to reducing the pulse width and lowering the reflected light power, the off-time of the acousto-optic devices must be sufficiently short. This requires precise testing of the off-time of the acousto-optic devices to pre-select those with short off-time.

[0003] Currently, the turn-off time of acousto-optic devices is typically tested by measuring the fall time of the pulsed light. Testing equipment includes... Figure 1 As shown, the signal source provides a pulse signal to the acousto-optic device. The continuous laser emitted by the laser is modulated into pulsed light by the acousto-optic device. The photodetector receives the pulsed light signal, and the waveform is tested by an oscilloscope. By using the oscilloscope's function of calculating the waveform fall time, the time it takes for the waveform to fall from 90% of its amplitude to 10% can be automatically calculated and displayed as the turn-off time of the acousto-optic device.

[0004] The main problem with this device is that, due to its limited detection capability, the minimum measurable optical power is typically in the hundreds of nW range, while wind lidar can measure signal light with power in the pW range. Furthermore, this method suffers from low detection sensitivity, and the pulse fall time calculated by the oscilloscope is short, failing to fully reflect the actual turn-off time of the acousto-optic devices when applied to wind lidar. Therefore, a more sensitive method that can accurately measure the turn-off time of acousto-optic devices in wind lidar systems is needed. Utility Model Content

[0005] To address the problems of existing technologies, this invention provides a device for testing the turn-off time of acousto-optic devices. This device can amplify the pulsed light tail signal and detect tail signals with power at the pW level, effectively improving detection sensitivity and enabling more accurate testing of acousto-optic turn-off time.

[0006] This utility model provides an acousto-optic device off-time testing device, including an acousto-optic device connected to a signal source and a laser. The continuous laser emitted by the laser is split into two beams by a beam splitter, and both beams are directed toward a coupler. The acousto-optic device is placed in the optical path of one of the beams. The coupler is sequentially connected to a balanced detector, a signal acquisition system, and a signal processing system.

[0007] As a further improvement, the coupler is a 3dB coupler.

[0008] In a further improvement, an optical attenuator is also provided in the optical path where the acousto-optic device is located. The laser in this optical path passes through the acousto-optic device and the optical attenuator in sequence before entering the coupler.

[0009] In a further improvement, the coupler receives two beams of light, including the local oscillator light directly transmitted by the beam splitter and the frequency-shifting pulsed light modulated by the acousto-optic device.

[0010] In a further improvement, the pulsed light and the local oscillator light are mixed by a coupler and then beat on the surface of the balanced detector.

[0011] The working method of this utility model is as follows:

[0012] The signal source provides pulse signals to the acousto-optic device. The laser emits continuous laser light, which is split into two beams after passing through a beam splitter. One beam is used as the local oscillator light, and the other beam is modulated into pulse light and frequency shifted after passing through the acousto-optic device. Then, it is mixed with the local oscillator light through a 3 dB coupler and beats on the surface of the balanced detector. The signal acquisition system collects the beat frequency signal, and after signal processing, the off-time of the acousto-optic device is tested.

[0013] The beneficial effects of this invention are as follows: It utilizes an optical attenuator to attenuate the power of the pulsed light modulated by the acousto-optic device, equivalent to the reflected light in a wind-measuring lidar. Based on the principle of coherent detection, by beating with the local oscillator, the signal during the acousto-optic device's turn-off process can be amplified, and the detectable signal power can reach the pW level, effectively improving detection sensitivity. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of this utility model, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of the structure of an existing acousto-optic device turn-off time testing device.

[0016] Figure 2 This is a schematic diagram of the structure of the acousto-optic device off-time testing device of this utility model.

[0017] Figure 3 This is a flowchart of a signal processing system.

[0018] Figure 4 This is a schematic diagram for calculating the starting point of time.

[0019] Figure 5 This is a schematic diagram for calculating the shutdown time. Detailed Implementation

[0020] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present utility model.

[0021] This invention provides a device for testing the off-time of acousto-optic devices, such as... Figure 2As shown, the device includes an acousto-optic device connected to a signal source and a laser. The continuous laser emitted by the laser is split into two beams by a beam splitter, both beams pointing towards a coupler. The acousto-optic device and an optical attenuator are sequentially arranged in the optical path of one of the beams. The coupler is sequentially connected to a balanced detector, a signal acquisition system, and a signal processing system. Based on the principle of coherent detection, this invention mixes the local oscillator light with the pulsed light modulated by the acousto-optic device through the coupler, and then beats the signal on the surface of the balanced detector. This amplifies the tail signal of the pulsed light, enabling the detection of tail signals with pW-level power, effectively improving detection sensitivity and achieving more accurate testing of the acousto-optic turn-off time.

[0022] As a further improvement, the coupler is a 3dB coupler.

[0023] In a further improvement, the coupler receives two beams of light, including the local oscillator light directly transmitted by the beam splitter and the frequency-shifting pulsed light modulated by the acousto-optic device.

[0024] In a further improvement, the pulsed light and the local oscillator light are mixed by a coupler and then beat on the surface of the balanced detector.

[0025] Based on this device, the following technical effects can also be achieved:

[0026] 1. By performing Fourier transform on the beat frequency signal at each moment, the time domain performance of the acousto-optic device during the turn-off process is converted to the frequency domain. By using frequency domain characterization, signals exceeding the time domain detection limit can be tested, thus improving detection sensitivity.

[0027] 2. By calculating the signal-to-noise ratio (SNR) of the acousto-optic device at each moment during its turn-off process, and combining this with the SNR detection threshold of the wind-measuring lidar, the time required for the acousto-optic device to completely turn off is obtained. This allows for the calculation of the acousto-optic turn-off time, suitable for practical applications of wind-measuring lidar, and enables accurate testing of the turn-off time of acousto-optic devices. This method can be used to screen acousto-optic devices with short turn-off times, thereby reducing the blind zone of the wind-measuring lidar.

[0028] The working method of this utility model is as follows:

[0029] The signal source provides pulse signals to the acousto-optic device. The laser emits continuous laser light, which is split into two beams by a beam splitter. One beam serves as the local oscillator, while the other beam is modulated into pulsed light and frequency-shifted after passing through the acousto-optic device. The light power is then attenuated to the level of the reflected light from the internal surface of the wind-measuring lidar system by an optical attenuator. This attenuated light is then mixed with the local oscillator light via a 3 dB coupler, and beats at the surface of the balanced detector. The signal acquisition system collects the beat signal and then performs signal processing on the acquired time-domain signal.

[0030] Those skilled in the art can, according to, Figure 3The process shown processes the signal, and one specific implementation is as follows.

[0031] (1) Calculation of the starting point of time: such as Figure 4 As shown in the figure, the absolute value of the acquired time-domain signal voltage is calculated, and then the average value is accumulated to obtain the pulse envelope of the beat frequency signal, as shown in the figure below. The maximum voltage Vm is calculated, and when the voltage of the pulse drop portion drops to 90% of the maximum value Vm, this moment is defined as the start time t0 of the acousto-optic device being turned off.

[0032] (2) Time gate division: Set the calculation time range after the acousto-optic device starts to turn off, and divide the time gate within the calculation range according to the required turn-off time resolution.

[0033] (3) Signal spectrum calculation: Perform Fourier transform on the time-domain signal at each moment, accumulate and denoise to obtain the signal spectrum at each moment after the acousto-optic signal begins to turn off. This process transforms the time-domain representation of the acousto-optic turn-off time to the frequency domain through Fourier transform. Using the frequency domain to characterize the turn-off process of the acousto-optic device can effectively solve the problem of limited detection capability when using the time-domain method. It can test signals that exceed the time-domain detection limit and improve the sensitivity of the test.

[0034] (4) Noise spectrum calculation: Perform Fourier transform on the time domain signal at an infinite time, accumulate and denoise to obtain the noise spectrum.

[0035] (5) Signal-to-noise ratio calculation: Based on the signal spectrum at each time point, peak values ​​are found, and the intensity corresponding to the peak value in the spectrum is denoted as I. S Then, take the intensity corresponding to that frequency in the noise spectrum, and denote it as I. N The formula for calculating the signal-to-noise ratio is:

[0036]

[0037] The signal-to-noise ratio (SNR) can be obtained from the formula for calculating the SNR at various times after the acousto-optic device is turned off.

[0038] (6) Turn-off time calculation: Based on the calculated signal-to-noise ratio data, plot the signal-to-noise ratio-time curve, such as... Figure 5 As shown in the figure, the signal-to-noise ratio (SNR) gradually decreases with increasing time, representing the gradual weakening of the signal after the acousto-optic device is turned off. To ensure the detection sensitivity of the wind-measuring lidar while reducing the false alarm rate, the radar system typically sets a SNR detection threshold. Signals above the threshold are detected, while signals below the threshold are considered invalid. After the acousto-optic device is turned off, when the signal-to-noise ratio drops below the radar detection threshold, it is considered an invalid signal and will not interfere with the system's detection. This moment is the time when the acousto-optic device is completely turned off, denoted as t1. The turn-off time of the acousto-optic device is...

[0039] .

[0040] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on its differences from other embodiments. In particular, for the device embodiments, the above descriptions are merely preferred embodiments of this utility model. Since they are fundamentally similar to the method embodiments, the descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments. The above descriptions are merely specific embodiments of this utility model, but the protection scope of this utility model is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this utility model, without departing from the principle of this utility model, should be included within the protection scope of this utility model. Therefore, the protection scope of this utility model should be determined by the scope of the claims.

Claims

1. An apparatus for testing the turn-off time of an acousto-optic device, comprising an acousto-optic device connected with a signal source and a laser, characterized in that: The continuous laser emitted by the laser is split into two beams of light by a beam splitter, both of which are directed towards a coupler, and an acousto-optic device is arranged in the light path of one of the two beams of light; the coupler is connected in sequence with a balanced detector, a signal acquisition system and a signal processing system.

2. The test apparatus of claim 1, wherein: The coupler is a 3dB coupler.

3. The test apparatus of claim 1, wherein: An optical attenuator is further arranged in the light path of the acousto-optic device, and the laser in the light path sequentially passes through the acousto-optic device and the optical attenuator before entering the coupler.

4. The apparatus of claim 1, wherein: The two beams of light received by the coupler include the local oscillator light directly sent by the beam splitter and the frequency-shifted pulsed light generated by the acousto-optic device.

5. The apparatus of claim 4, wherein: The pulsed light and the local oscillator light are mixed by the coupler to generate beat frequency on the surface of the balanced detector.

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