Chirped pulse optical frequency sweeping range adjusting module and chirped pulse optical time domain reflectometer

Abstract

The invention provides a chirped pulse optical sweep frequency range adjusting module and a chirped pulse optical time domain reflectometer, which comprise a waveform generator, a first modulator and a second modulator, the first modulator and the second modulator are connected in series, and the waveform generator provides two paths of chirped radio frequency signals generated by the waveform generator to the first modulator and the second modulator respectively. After the first modulator receives a laser signal, the first modulator modulates the laser signal according to one path of chirp radio frequency signal and outputs positive or negative first-order diffraction light generated after modulation to the second modulator, and the second modulator modulates the positive or negative first-order diffraction light according to the other path of chirp radio frequency signal and outputs the positive or negative first-order diffraction light to the second modulator. Negative or positive primary diffraction light generated after modulation is taken as chirped pulse light to be output, the frequency shift of the initial frequency of the chirped pulse light is the difference of the initial frequencies of the two paths of chirped radio frequency signals, and the frequency sweeping bandwidth is the sum of the bandwidths of the two paths of chirped radio frequency signals. According to the invention, the initial sweep frequency and the sweep bandwidth of the chirped pulse light can be adjusted.

Description

Technical Field

[0001] The present invention belongs to the field of chirped pulse light generation, and particularly relates to a chirped pulse light sweep range adjustment module and a chirped pulse optical time domain reflectometer. Background Art

[0002] Distributed optical fiber sensing technology uses the scattering effect in optical fibers to measure external disturbances. Among them, the chirped pulse optical time domain reflectometer (CP-OTDR) based on Rayleigh scattering can measure external vibrations, strains, and temperature changes. Different from traditional single-frequency pulses, CP-OTDR linearly modulates the frequency of pulses through a modulator. Its performance indicators are related to the sweep range of the pulses. Increasing the sweep range can improve indicators such as spatial resolution and dynamic range. CP-OTDR usually achieves pulse frequency sweeping through an acousto-optic modulator (AOM). However, limited by the resonance characteristics of the transducer material inside the AOM, its working bandwidth at the center frequency is limited, which restricts the sweep range of CP-OTDR and makes it difficult to further improve performance such as spatial resolution. Summary of the Invention

[0003] The present invention provides a chirped pulse light sweep range adjustment module and a chirped pulse optical time domain reflectometer to solve the problems that the sweep range of chirped pulse light is currently limited and the performance of chirped pulse optical time domain reflection is difficult to be further improved.

[0004] According to the first aspect of the embodiments of the present invention, a chirped pulse light sweep range adjustment module is provided, including a waveform generator and a cascaded modulator composed of a first modulator and a second modulator connected in series. Two output terminals of the waveform generator are respectively connected to the first modulator and the second modulator;

[0005] The waveform generator provides two generated chirped radio frequency signals to the first modulator and the second modulator respectively. After receiving the laser signal, the first modulator modulates the laser signal according to one chirped radio frequency signal, and outputs the generated positive or negative first-order diffracted light to the second modulator. The second modulator modulates the received positive or negative first-order diffracted light according to the other chirped radio frequency signal, and takes the generated negative or positive first-order diffracted light as the chirped pulse light output. The frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped radio frequency signals, and the sweep bandwidth is the sum of the bandwidths of the two chirped radio frequency signals.

[0006] Optionally, there are multiple cascade modulators, each of which is connected in series, and in this case, a first splitter and a second splitter are also included, one output end of the waveform generator is respectively connected to the first modulator in each cascade modulator through the first splitter, and the other output end is respectively connected to the second modulator in each cascade modulator through the second splitter;

[0007] The waveform generator provides one chirped RF signal generated by it to the first modulator in each cascade modulator through the first splitter, and provides another chirped RF signal generated by it to the second modulator in each cascade modulator through the second splitter;

[0008] The last cascade modulator outputs a chirped pulse light, the frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped RF signals multiplied by the number of the cascade modulators; the sweep bandwidth is the sum of the bandwidths of the two chirped RF signals multiplied by the number of the cascade modulators.

[0009] Optionally, the first modulator and the second modulator in the cascade modulator are both acousto-optic modulators, and both include a piezoelectric transducer and an acousto-optic crystal, the piezoelectric transducer converts the received corresponding chirped radio frequency signal from electrical energy to mechanical energy to generate ultrasonic waves, and applies the ultrasonic waves to the acousto-optic crystal, and during the propagation of the ultrasonic waves along the acousto-optic crystal, the acousto-optic crystal modulates the incident light received by it;

[0010] During the propagation of the ultrasonic wave along the acousto-optic crystal, a dynamic grating is formed in the acousto-optic crystal, and the grating period of the dynamic grating is equal to the wavelength of the ultrasonic wave; the grating on the acousto-optic crystal where the light is incident is defined as an incident grating, the side of the incident grating facing the piezoelectric transducer is defined as the front side, and the side away from the piezoelectric transducer is defined as the back side, the direction perpendicular to the propagation direction of the ultrasonic wave is defined as the width direction, and the middle position of the incident grating in the width direction is defined as the incident point on the incident grating;

[0011] For the first modulator, its incident light is incident to the incident point on the front side or the back side of the incident grating on the acousto-optic crystal, so that the first modulator outputs its positive or negative first-order diffraction light to the second modulator;

[0012] For the second modulator, the positive or negative first-order diffraction light is incident on the incident point on the back side or the front side of the incident grating on the acousto-optic crystal, so that the second modulator outputs negative or positive first-order diffraction light.

[0013] Optionally, the incident angle of the incident light incident on the acousto-optic crystal in the first modulator and the incident angle of the incident light incident on the acousto-optic crystal in the second modulator are equal and both satisfy the Bragg diffraction condition, and the incident angle θ is expressed as:

[0014]

[0015] Where λ is the wavelength of the incident light, n represents the refractive index of the acousto-optic crystal, v represents the sound velocity, and f represents the center frequency of the corresponding modulator, that is, the center frequency of its corresponding chirped RF signal.

[0016] Optionally, the distance H between the incident gratings of the first modulator and the second modulator in the ultrasonic propagation direction is: H = (W1 + W2 + 2*W0)*tanθ / 2, where W1 represents the width of the acousto-optic crystal in the first modulator, W2 represents the width of the acousto-optic crystal in the second modulator, W0 represents the distance between the first modulator and the second modulator in the width direction, and θ represents the incident angle.

[0017] Optionally, the initial frequency positive and negative signs of the two chirped RF signals are the same, and the frequency linear change slope signs are opposite. The frequency linear change slope of the chirped pulse light is equal to the sum of the absolute values of the frequency linear change slopes of the two chirped RF signals.

[0018] Optionally, the difference between the initial frequencies of the two chirped RF signals is 0.

[0019] Optionally, it further includes drive amplifiers. The two chirped RF signals generated by the waveform generator are respectively processed by the corresponding drive amplifiers and then provided to the first modulator and the second modulator.

[0020] According to the second aspect of the embodiments of the present invention, a chirped pulse optical time domain reflectometer is provided, including a laser, a circulator, a fiber under test, a photodetector, a data processor, and the chirped pulse light sweep range adjustment module according to any one of claims 1 to 8. The laser is connected to the first end of the circulator through the chirped pulse light sweep range adjustment module. The second end of the circulator is connected to the fiber under test, and the third end is connected to the data processor through the photodetector.

[0021] Optionally, it further includes a splitter and a coupler. The splitter is arranged between the laser and the chirped pulse light sweep range adjustment module. The first output end of the splitter is connected to the chirped pulse light sweep range adjustment module, the second output end is connected to the second input end of the coupler, the third end of the circulator is connected to the first input end of the coupler, and the output end of the coupler is connected to the photodetector.

[0022] The beneficial effects of the present invention are:

[0023] 1. In the present invention, two chirped radio frequency signals with the same positive and negative signs of the initial frequencies are respectively provided to a first modulator and a second modulator. The first modulator and the second modulator are connected in series to form a cascaded modulator. After the laser signal is transmitted to the first modulator in the cascaded modulator, the first modulator modulates the laser signal according to one of the chirped radio frequency signals, and transmits the positive or negative first-order diffracted light generated by the modulation to the second modulator. The second modulator modulates the positive or negative first-order diffracted light according to the other chirped radio frequency signal, and outputs the negative or positive first-order diffracted light as the chirped pulse light. Since the positive and negative signs of the first-order diffracted light output by the first modulator and the second modulator are opposite, one positive and one negative can cancel each other out. Therefore, the frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped radio frequency signals. By adjusting the relative magnitudes of the initial frequencies of the two chirped radio frequency signals, the magnitude of the frequency shift of the initial frequency of the chirped pulse light can be adjusted, and the intermediate frequency shift is shifted to a low frequency shift near the direct current. When detecting the signal, it can be ensured that the chirped pulse light sweeps the frequency starting from the direct current, so that the swept frequency signal fills the entire detection bandwidth, and the utilization rate of the detection bandwidth reaches the limit. The swept frequency bandwidth of the chirped pulse light in the present invention is the sum of the bandwidths of the two chirped radio frequency signals. By adjusting the bandwidths of the two chirped radio frequency signals, the swept frequency bandwidth of the chirped pulse light can be adjusted, which breaks through the swept frequency bandwidth limitation of a single modulator and greatly expands the swept frequency bandwidth. In addition, the extinction ratio of the cascaded modulator in the present invention is the sum of the extinction ratios of each internal modulator, which breaks through the extinction ratio limitation of a single modulator. The bandwidth of the chirped radio frequency signal generated by the waveform generator in the present invention can be less than the swept frequency bandwidth of the chirped pulse light output after the final modulation, which reduces the bandwidth and cost requirements when generating the chirped radio frequency signal.

[0024] 2. The present invention uses multiple cascaded modulators, provides the same chirped radio frequency signal to the first modulator in each cascaded modulator, and provides the same other chirped radio frequency signal to the second modulator in each cascaded modulator. Thus, the frequency shift of the initial frequency of the chirped pulse light output by each cascaded modulator is the difference between the initial frequencies of the two chirped radio frequency signals, and the swept frequency bandwidth is the sum of the bandwidths of the two chirped radio frequency signals. Thereafter, when the multiple cascaded modulators are connected in series, the swept frequency bandwidth of the output chirped pulse light can be increased multiplicatively. In addition, the present invention uses two splitters to respectively provide the two radio frequency signals to the corresponding modulators in each cascaded modulator, which can reduce the channel number requirements of the waveform generator, make the structure of the waveform generator required to generate the chirped radio frequency signal simpler, and reduce the module complexity, cost and occupied space.

[0025] 3. By determining the incident points and incident angles of the incident light on the first modulator and the second modulator, and adjusting the relative positional relationship between the first modulator and the second modulator, the present invention can ensure that the light output from both the first modulator and the second modulator is first-order diffracted light, and the positive and negative signs of the two first-order diffracted lights are opposite;

[0026] 4. By making the initial frequency difference between the two chirped radio frequency signals of the present invention be 0, the intermediate frequency shift of the chirped pulsed light can be shifted to a low frequency shift near the direct current. When detecting the signal, it can be ensured that the chirped pulsed light sweeps the frequency starting from the direct current, so that the swept frequency signal fills the entire detection bandwidth, and the utilization rate of the detection bandwidth reaches the limit;

[0027] 5. Before the two chirped radio frequency signals generated by the waveform generator of the present invention are respectively provided to the first modulator and the second modulator, they are also processed by a driving amplifier, thereby ensuring the modulation effect;

[0028] 6. By applying the chirped pulsed light sweep frequency range adjustment module of the present invention to a pulsed light time domain reflectometer, since the spatial resolution of the reflectometer is proportional to the sweep bandwidth, when the pulsed light time domain reflectometer uses the chirped pulsed light sweep frequency range adjustment module to increase its sweep bandwidth, its performance indicators such as spatial resolution and measurement parameter dynamic range are also improved. Description of the Drawings

[0029] Figure 1 is a schematic structural diagram of an embodiment of the chirped pulsed light sweep frequency range adjustment module of the present invention;

[0030] Figure 2 is a schematic diagram of the frequency modulation process of a single cascaded modulator;

[0031] Figure 3 is a schematic diagram of the relative positional relationship between the first modulator and the second modulator in the cascaded modulator;

[0032] Figure 4 is a schematic structural diagram of another embodiment of the chirped pulsed light sweep frequency range adjustment module of the present invention;

[0033] Figure 5 is a schematic diagram of the frequency modulation process of multiple cascaded modulators connected in series of the present invention;

[0034] Figure 6 is a schematic structural diagram of an embodiment of the chirped pulsed light time domain reflectometer of the present invention;

[0035] Figure 7 is a schematic structural diagram of another embodiment of the chirped pulsed light time domain reflectometer of the present invention. Detailed Embodiments

[0036] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above-mentioned objects, features, and advantages of the embodiments of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0037] In the description of the present invention, unless otherwise specified and defined, it should be noted that the term "connection" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the communication inside two components. It can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific situations.

[0038] See Figure 1 , which is a schematic structural diagram of an embodiment of the chirped pulse optical frequency sweep range adjustment module of the present invention. The chirped pulse optical frequency sweep range adjustment module may include a waveform generator and a cascade modulator composed of a first modulator and a second modulator connected in series. Two output terminals of the waveform generator are respectively connected to the first modulator and the second modulator. The waveform generator provides two generated chirped radio frequency signals to the first modulator and the second modulator respectively. After receiving the laser signal, the first modulator modulates the laser signal according to one chirped radio frequency signal and outputs the generated positive or negative first-order diffracted light to the second modulator. The second modulator modulates the received positive or negative first-order diffracted light according to the other chirped radio frequency signal and takes the generated negative or positive first-order diffracted light as the chirped pulse light output. The frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped radio frequency signals, and the frequency sweep bandwidth is the sum of the bandwidths of the two chirped radio frequency signals.

[0039] In this embodiment, the laser signal can be provided by a narrow linewidth laser. As shown in Figure 2 , the two chirped radio frequency signals generated by the waveform generator are respectively provided to two modulators. The modulation bandwidths of the modulators are [f 10 , f 11 and [f 20 , f 21 respectively. Taking the case where the first modulator outputs positive first-order diffracted light and the second modulator outputs negative first-order diffracted light as an example, then f 10 >0, f 11 >0, f 20 <0, f 21 <0. The two chirped radio frequency signals RF 1 (t) and RF 2 (t) can be respectively expressed as:

[0040]

[0041] Where A is the amplitude of two chirped RF signals, rect() is the matrix function, t is the time variable, T is the duration of the chirped RF signal, f 10 and |f 20 | are the initial frequencies of two chirped RF signals respectively, and the positive and negative signs of the two initial frequencies are the same. is the RF 1 (t) sweep rate, and the frequency increases linearly; is the RF 2 (t) sweep rate, and the frequency decreases linearly. The signs of the frequency linear change slopes of the two chirped RF signals are opposite.

[0042] The laser signal E(t) can be expressed as:

[0043] E(t) = exp(j2πf c t)

[0044] where f c is the initial frequency of the laser signal.

[0045] After the two chirped RF signals are respectively provided to two modulators, the two modulators respectively modulate the incident light according to the corresponding chirped RF signals. After being modulated by the first modulator, the laser signal can be expressed as:

[0046]

[0047] where is the phase modulation term of the incident light by the first modulator, and A 1 is the amplitude of the laser signal after being modulated by the first modulator. The frequency shift - time relationship of this phase modulation term is as shown in Figure 2 (a). The positive first - order diffracted light in the laser signal modulated by the first modulator is transmitted to the second modulator, and after being modulated by the second modulator, it can be expressed as:

[0048]

[0049] where is the phase modulation term of the incident light by the second modulator, and A 0 is the amplitude of the laser signal after being modulated by the first modulator and the second modulator. The frequency shift - time relationship of this phase modulation term is as shown in Figure 2 (b). f 0 = f 10 + f 20 , f 1 = f 11 + f 21 , k 0 = (f 1-f 0 ) / T. It can be seen that after the laser signal is modulated by the cascade modulator, the first modulator outputs positive or negative first-order diffraction light, and the second modulator outputs negative or positive first-order diffraction light, so that the initial frequency signs of the two modulators are opposite, so that the frequency shift of the initial frequency of the formed chirped pulse light is the sum of the initial frequencies of the two modulators, that is, the difference between the initial frequencies of the two chirped RF signals, and the frequency sweep bandwidth is the sum of the bandwidths of the two chirped RF signals, such as Figure 2 (c) as shown.

[0050] In this embodiment, the initial frequencies of the two chirped RF signals have the same positive and negative signs, and the linear frequency change slopes have opposite signs. The linear frequency change slope of the chirped pulse light can be equal to the sum of the absolute values ​​of the linear frequency change slopes of the two chirped RF signals. Among them, the difference between the initial frequencies of the two chirped RF signals can be 0, and the absolute values ​​of the linear frequency change slopes can be the same. The present invention makes the sum of the initial frequencies of the two chirped RF signals 0, and can move the intermediate frequency shift of the chirped pulse light to the low frequency shift near DC. When detecting the signal, it can ensure that the chirped pulse light starts the frequency sweep from DC, so that the frequency sweep signal can fill the entire detection bandwidth, so that the utilization rate of the detection bandwidth reaches the limit.

[0051] In addition, combined Figure 3 As shown, the first modulator and the second modulator in the cascade modulator can both be acousto-optic modulators, and both include a piezoelectric transducer and an acousto-optic crystal. The piezoelectric transducer converts the received corresponding chirped RF signal from electrical energy into mechanical energy to generate ultrasonic waves, and applies the ultrasonic waves to the acousto-optic crystal. During the propagation of the ultrasonic waves along the acousto-optic crystal, the acousto-optic crystal modulates the incident light received by it.

[0052] During the propagation of the ultrasonic wave along the acousto-optic crystal, a dynamic grating is formed in the acousto-optic crystal, and the grating period of the dynamic grating is equal to the wavelength of the ultrasonic wave; let the grating on the acousto-optic crystal where the light is incident be the incident grating (for example, it can be the central grating of the dynamic grating in the ultrasonic wave propagation direction), the side of the incident grating facing the piezoelectric transducer is the front side, and the side away from the piezoelectric transducer is the back side, let the direction perpendicular to the ultrasonic wave propagation direction be the width direction, and the middle position of the incident grating in the width direction is the incident point on the incident grating. For the first modulator, its incident light is incident to the incident point on the front side or back side of the incident grating on its acousto-optic crystal, so that the first modulator outputs its positive or negative first-order diffraction light to the second modulator; for the second modulator, the positive or negative first-order diffraction light is incident to the incident point on the back side or front side of the incident grating on its acousto-optic crystal, so that the second modulator outputs negative or positive first-order diffraction light.

[0053] Among them, the incident angles of the incident light on the acousto-optic crystal in the first modulator and the incident angles of the incident light on the acousto-optic crystal in the second modulator can be equal and both satisfy the Bragg diffraction condition. The incident angle θ can be expressed as:

[0054]

[0055] where λ is the wavelength of the incident light, n represents the refractive index of the acousto-optic crystal, v represents the sound velocity, and f represents the center frequency of the corresponding modulator, that is, the center frequency of its corresponding chirped radio frequency signal.

[0056] The distance H between the incident gratings of the first modulator and the second modulator in the ultrasonic wave propagation direction can be: H = (W1 + W2 + 2 * W0) * tanθ / 2, where W1 represents the width of the acousto-optic crystal in the first modulator, W2 represents the width of the acousto-optic crystal in the second modulator, W0 represents the distance between the first modulator and the second modulator in the width direction, and θ represents the incident angle. By determining the incident points and incident angles of the incident light on the first modulator and the second modulator and adjusting the relative position relationship between the first modulator and the second modulator, the present invention can ensure that the outputs of both the first modulator and the second modulator are first-order diffracted lights, and the positive and negative signs of the two first-order diffracted lights are opposite.

[0057] As can be seen from the above embodiments, the present invention provides two chirped radio frequency signals with the same positive and negative signs of the initial frequencies to a first modulator and a second modulator respectively, and the first modulator and the second modulator are connected in series to form a cascaded modulator. After the laser signal is transmitted to the first modulator in the cascaded modulator, the first modulator modulates the laser signal according to one corresponding chirped radio frequency signal, and transmits the positive or negative first-order diffracted light generated by the modulation to the second modulator. The second modulator modulates the positive or negative first-order diffracted light according to the other chirped radio frequency signal, and outputs the negative or positive first-order diffracted light as the chirped pulse light. Since the positive and negative signs of the first-order diffracted light output by the first modulator and the second modulator are opposite, one positive and one negative can cancel each other out. Therefore, the frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped radio frequency signals. By adjusting the relative magnitudes of the initial frequencies of the two chirped radio frequency signals, the magnitude of the frequency shift of the initial frequency of the chirped pulse light can be adjusted, and the intermediate frequency shift is shifted to a low frequency shift near the direct current. When detecting the signal, it can be ensured that the chirped pulse light sweeps the frequency starting from the direct current, so that the swept frequency signal fills the entire detection bandwidth, and the utilization rate of the detection bandwidth reaches the limit. The swept frequency bandwidth of the chirped pulse light of the present invention is the sum of the bandwidths of the two chirped radio frequency signals. By adjusting the bandwidths of the two chirped radio frequency signals, the swept frequency bandwidth of the chirped pulse light can be adjusted, which breaks through the swept frequency bandwidth limitation of a single modulator and greatly expands the swept frequency bandwidth. In addition, the extinction ratio of the cascaded modulator of the present invention is the sum of the extinction ratios of each internal modulator, which breaks through the extinction ratio limitation of a single modulator. The bandwidth of the chirped radio frequency signal generated by the waveform generator of the present invention can be less than the swept frequency bandwidth of the chirped pulse light output after the final modulation, which reduces the bandwidth and cost requirements when generating the chirped radio frequency signal.

[0058] See Figure 4 , which is a schematic structural diagram of another embodiment of the chirped pulse light swept frequency range adjustment module of the present invention. Figure 4 Differing from the embodiment shown in Figure 1 , in this chirped pulse light swept frequency range adjustment module, there are multiple cascaded modulators, and each cascaded modulator is connected in series. At this time, it also includes a first splitter and a second splitter. One output terminal of the waveform generator is respectively connected to the first modulators in each cascaded modulator through the first splitter, and the other output terminal is respectively connected to the second modulators in each cascaded modulator through the second splitter. The waveform generator provides one chirped radio frequency signal generated by it to the first modulators in each cascaded modulator through the first splitter, and provides the other chirped radio frequency signal generated by it to the second modulators in each cascaded modulator through the second splitter. Among them, in combination with Figure 5As shown, the last cascaded modulator outputs chirped pulsed light. The frequency shift of the initial frequency of the chirped pulsed light is the difference between the initial frequencies of the two chirped RF signals, multiplied by the number of cascaded modulators; the swept frequency bandwidth is the sum of the bandwidths of the two chirped RF signals, multiplied by the number of cascaded modulators. Figure 4 Taking two cascaded modulators in series as an example.

[0059] The present invention uses multiple cascaded modulators. The same chirped RF signal is provided to the first modulator in each cascaded modulator, and the same other chirped RF signal is provided to the second modulator in each cascaded modulator. Thus, the frequency shift of the initial frequency of the chirped pulsed light output by each cascaded modulator can be the difference between the initial frequencies of the two chirped RF signals, and the swept frequency bandwidth can be the sum of the bandwidths of the two chirped RF signals. After that, multiple cascaded modulators are cascaded in series, which can multiply the swept frequency bandwidth of the output chirped pulsed light; in addition, the present invention uses two splitters to respectively provide the two RF signals to the corresponding modulators in each cascaded modulator, which can reduce the channel number requirement of the waveform generator, make the structure of the waveform generator required to generate chirped RF signals simpler, and reduce the module complexity, cost and occupied space.

[0060] Figure 4 And Figure 1 The difference from the embodiment shown is that the chirped pulsed light swept frequency range adjustment module may further include a driver amplifier. The two chirped RF signals generated by the waveform generator are respectively processed by the corresponding driver amplifiers and then provided to the first modulator and the second modulator. Before the two chirped RF signals generated by the waveform generator of the present invention are respectively provided to the first modulator and the second modulator, they are also processed by the driver amplifier, which can ensure the modulation effect.

[0061] As can be seen from the above embodiments, the present invention provides two chirped radio frequency signals with the same positive and negative signs of the initial frequencies to a first modulator and a second modulator respectively, and the first modulator and the second modulator are connected in series to form a cascaded modulator. After the laser signal is transmitted to the first modulator in the cascaded modulator, the first modulator modulates the laser signal according to one of the chirped radio frequency signals, and transmits the positive or negative first-order diffracted light generated by the modulation to the second modulator. The second modulator modulates the positive or negative first-order diffracted light according to the other chirped radio frequency signal, and outputs the negative or positive first-order diffracted light as the chirped pulse light. Since the positive and negative signs of the first-order diffracted light output by the first modulator and the second modulator are opposite, one positive and one negative can cancel each other out. Therefore, the frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped radio frequency signals. By adjusting the relative magnitudes of the initial frequencies of the two chirped radio frequency signals, the magnitude of the frequency shift of the initial frequency of the chirped pulse light can be adjusted, and the intermediate frequency shift is shifted to a low frequency shift near the direct current. When detecting the signal, it can be ensured that the chirped pulse light sweeps the frequency starting from the direct current, so that the swept frequency signal fills the entire detection bandwidth, and the utilization rate of the detection bandwidth reaches the limit. The swept frequency bandwidth of the chirped pulse light of the present invention is the sum of the bandwidths of the two chirped radio frequency signals. By adjusting the bandwidths of the two chirped radio frequency signals, the swept frequency bandwidth of the chirped pulse light can be adjusted, which breaks through the swept frequency bandwidth limitation of a single modulator and greatly expands the swept frequency bandwidth. In addition, the extinction ratio of the cascaded modulator of the present invention is the sum of the extinction ratios of each internal modulator, which breaks through the extinction ratio limitation of a single modulator. The bandwidth of the chirped radio frequency signal generated by the waveform generator of the present invention can be smaller than the swept frequency bandwidth of the chirped pulse light output after the final modulation, which reduces the bandwidth and cost requirements when generating the chirped radio frequency signal.

[0062] See Figure 6 , which is a schematic structural diagram of an embodiment of the chirped pulse optical time domain reflectometer of the present invention. The chirped pulse optical time domain reflectometer may include a laser, a circulator, a fiber under test, a photodetector, a data processor, and the above-mentioned chirped pulse optical swept frequency range adjustment module. The laser is connected to the first end of the circulator through the chirped pulse optical swept frequency range adjustment module. The second end of the circulator is connected to the fiber under test, and the third end is connected to the data processor through the photodetector. In this embodiment, frequency shift demodulation is adopted, so a direct detection system is used. The spatial resolution of the chirped pulse optical time domain reflectometer can be expressed as:

[0063]

[0064] Δz is the spatial resolution, C is the speed of light, n is the refractive index, and B is the swept frequency bandwidth.

[0065] As can be seen from the above embodiments, the present invention applies the chirped pulse optical frequency sweep range adjustment module to the pulsed optical time domain reflectometer. Since the spatial resolution of the reflectometer is proportional to the frequency sweep bandwidth, when the pulsed optical time domain reflectometer uses the chirped pulse optical frequency sweep range adjustment module to increase its frequency sweep bandwidth, its performance indicators such as spatial resolution and dynamic range of measurement parameters are also improved.

[0066] See Figure 7 , which is a schematic structural diagram of another embodiment of the chirped pulse optical time domain reflectometer of the present invention. Figure 7 Differing from the Figure 6 shown embodiment, this chirped pulse optical time domain reflectometer may further include a beam splitter and a coupler. The beam splitter is disposed between the laser and the chirped pulse optical frequency sweep range adjustment module. The first output end of the beam splitter is connected to the chirped pulse optical frequency sweep range adjustment module, and the second output end is connected to the second input end of the coupler. The third end of the circulator is connected to the first input end of the coupler, and the output end of the coupler is connected to the photodetector. Phase demodulation is adopted in this embodiment, so a coherent detection system is used, and the photodetector can be a balanced detector. Figure 7 Differing from the Figure 6 shown embodiment, an optical amplifier is provided between the output end of the cascaded acousto-optic modulator and the first end of the circulator, and the data processor and the waveform generator can be synchronously triggered by a trigger source.

[0067] As can be seen from the above embodiments, the present invention applies the chirped pulse optical frequency sweep range adjustment module to the pulsed optical time domain reflectometer. Since the spatial resolution of the reflectometer is proportional to the frequency sweep bandwidth, when the pulsed optical time domain reflectometer uses the chirped pulse optical frequency sweep range adjustment module to increase its frequency sweep bandwidth, its performance indicators such as spatial resolution and dynamic range of measurement parameters are also improved.

[0068] It should be noted that: Figure 6 and Figure 7 in the chirped pulse optical time domain reflectometer shown, the chirped pulse optical frequency sweep range adjustment module can be not only Figure 1 the structure of a single cascaded modulator shown, but also Figure 4 the structure of multiple cascaded modulators connected in series shown.

[0069] Those skilled in the art will readily conceive of other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present invention, which follow the general principles of the present invention and include known common knowledge or conventional technical means in the technical field not disclosed by the present invention. The specification and embodiments are only regarded as exemplary, and the true scope and spirit of the present invention are pointed out by the following claims.

[0070] It should be understood that the present invention is not limited to the exact structures that have been described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only regulated by the appended claims.

Claims

1. A chirped pulse light sweep frequency range adjustment module, characterized in that: It comprises a waveform generator and a cascade modulator composed of a first modulator and a second modulator connected in series, wherein two output ends of the waveform generator are connected to the first modulator and the second modulator respectively; The waveform generator provides the two chirped RF signals it generates to the first modulator and the second modulator respectively. After receiving the laser signal, the first modulator modulates the laser signal according to one chirped RF signal, and outputs the positive or negative first-order diffraction light generated after modulation to the second modulator. The second modulator modulates the positive or negative first-order diffraction light it receives according to the other chirped RF signal, and takes the negative or positive first-order diffraction light generated after modulation as a chirped pulse light output. The frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped RF signals, and the sweep bandwidth is the sum of the bandwidths of the two chirped RF signals.

2. The chirped pulse light sweep frequency range adjustment module according to claim 1, characterized in that: There are multiple cascade modulators, each of which is connected in series. In this case, a first splitter and a second splitter are also included. One output end of the waveform generator is connected to the first modulator in each cascade modulator through the first splitter, and the other output end is connected to the second modulator in each cascade modulator through the second splitter. The waveform generator provides one chirped RF signal generated by it to the first modulator in each cascade modulator through the first splitter, and provides another chirped RF signal generated by it to the second modulator in each cascade modulator through the second splitter; The last cascade modulator outputs a chirped pulse light, the frequency shift of the initial frequency of the chirped pulse light is the difference between the initial frequencies of the two chirped RF signals multiplied by the number of the cascade modulators; the sweep bandwidth is the sum of the bandwidths of the two chirped RF signals multiplied by the number of the cascade modulators.

3. The chirped pulse light frequency sweep range adjustment module according to claim 1 or 2, characterized in that: The first modulator and the second modulator in the cascade modulator are both acousto-optic modulators, and both include a piezoelectric transducer and an acousto-optic crystal. The piezoelectric transducer converts the received corresponding chirped radio frequency signal from electrical energy to mechanical energy to generate ultrasonic waves, and applies the ultrasonic waves to the acousto-optic crystal. During the propagation of the ultrasonic waves along the acousto-optic crystal, the acousto-optic crystal modulates the incident light received by the acousto-optic crystal. During the propagation of the ultrasonic wave along the acousto-optic crystal, a dynamic grating is formed in the acousto-optic crystal, and the grating period of the dynamic grating is equal to the wavelength of the ultrasonic wave; the grating on the acousto-optic crystal where the light is incident is defined as an incident grating, the side of the incident grating facing the piezoelectric transducer is defined as the front side, and the side away from the piezoelectric transducer is defined as the back side, the direction perpendicular to the propagation direction of the ultrasonic wave is defined as the width direction, and the middle position of the incident grating in the width direction is defined as the incident point on the incident grating; For the first modulator, its incident light is incident to the incident point on the front side or the back side of the incident grating on the acousto-optic crystal, so that the first modulator outputs its positive or negative first-order diffraction light to the second modulator; For the second modulator, the positive or negative first-order diffraction light is incident on the incident point on the back side or the front side of the incident grating on the acousto-optic crystal, so that the second modulator outputs negative or positive first-order diffraction light.

4. The chirped pulse light frequency sweep range adjustment module according to claim 3, characterized in that: The incident angle of the incident light incident on the acousto-optic crystal in the first modulator and the incident angle of the incident light incident on the acousto-optic crystal in the second modulator are equal and both satisfy the Bragg diffraction condition. The incident angle θ is expressed as: Where λ is the wavelength of the incident light, n represents the refractive index of the acousto-optic crystal, v represents the speed of sound, and f represents the center frequency of the corresponding modulator, that is, the center frequency of the corresponding chirped RF signal.

5. The chirped pulse light frequency sweep range adjustment module according to claim 4, characterized in that: The distance H between the incident gratings of the first modulator and the second modulator in the ultrasonic propagation direction is: H = (W1+W2+2*W0)*tanθ / 2, wherein W1 represents the width of the acousto-optic crystal in the first modulator, W2 represents the width of the acousto-optic crystal in the second modulator, W0 represents the distance between the first modulator and the second modulator in the width direction, and θ represents the incident angle.

6. The chirped pulse light frequency sweep range adjustment module according to claim 1 or 2, characterized in that: The initial frequencies of the two chirped RF signals have the same positive and negative signs, and the linear frequency variation slopes have opposite signs. The linear frequency variation slope of the chirped pulse light is equal to the sum of the absolute values ​​of the linear frequency variation slopes of the two chirped RF signals.

7. The chirped pulse light sweep frequency range adjustment module according to claim 6, characterized in that: The difference between the initial frequencies of the two chirped radio frequency signals is 0.

8. The chirped pulse light frequency sweep range adjustment module according to claim 1 or 2, characterized in that: It also includes a driving amplifier. The two chirped radio frequency signals generated by the waveform generator are processed by corresponding driving amplifiers respectively and then provided to the first modulator and the second modulator.

9. A chirped pulse optical time domain reflectometer, characterized in that: The invention comprises a laser, a circulator, an optical fiber to be tested, a photodetector, a data processor and the chirped pulse light frequency sweep range adjustment module as described in any one of claims 1 to 8, wherein the laser is connected to a first end of the circulator through the chirped pulse light frequency sweep range adjustment module, a second end of the circulator is connected to the optical fiber to be tested, and a third end is connected to the data processor through the photodetector.

10. The chirped pulse optical time domain reflectometer according to claim 9, characterized in that: It also includes a spectrometer and a coupler, wherein the spectrometer is arranged between the laser and the chirped pulse light sweep frequency range adjustment module, the first output end of the spectrometer is connected to the chirped pulse light sweep frequency range adjustment module, the second output end is connected to the second input end of the coupler, the third end of the circulator is connected to the first input end of the coupler, and the output end of the coupler is connected to the photodetector.

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