A method and device for adaptively adjusting optical power for terahertz time-domain spectroscopy system

Abstract

The present invention discloses an optical power adaptive adjustment device and method for a terahertz time-domain spectroscopy system, which includes a femtosecond laser, a transmitting electrically controlled adjustable attenuator, a detecting electrically controlled adjustable attenuator, a transmitting optical splitter, a detecting optical splitter and an electrically controlled adjustable attenuator power control unit, an optical power conversion and transmission module, a main control MCU and a host computer PC; the main control MCU collects and analyzes the optical power signal sent by the optical power conversion and transmission module, compares it with the corresponding transmitting end optical power threshold or the detecting end optical power threshold, and outputs the corresponding pulse duty cycle signal to the electrically controlled adjustable attenuator power control unit, and the electrically controlled adjustable attenuator power control unit outputs voltage to the transmitting electrically controlled adjustable attenuator and the detecting electrically controlled adjustable attenuator based on the received pulse duty cycle signal. The EVOA is adaptively adjusted through the synergistic effect of the main control MCU and the electrically controlled adjustable attenuator power control unit, and the debugging is simple, the response speed is fast, the control accuracy is high, and the cost is low.

Description

Technical field:

[0001] The present invention belongs to the technical field of terahertz spectroscopy and imaging, and in particular relates to a method and a device for adaptively adjusting optical power for a terahertz time-domain spectroscopy system. Background technology:

[0002] Terahertz waves are electromagnetic waves with strong penetrating power and low photon energy between microwave and infrared bands. Their frequencies are between 0.1 and 10 THz (1 THz = 10 12 Hz). Terahertz spectroscopy and imaging systems use terahertz waves for photoconductive sampling and are widely used in material identification, safety inspection, non-destructive testing of materials and structures, in vivo examination of biological tissues, wireless communications and other fields.

[0003] During sampling, the laser generated by the femtosecond laser excites the photoconductive antenna to generate photogenerated carriers. These carriers form a current under the action of the terahertz pulse electric field. The terahertz pulse can be measured by detecting the change in the current. In order to achieve good detection accuracy and avoid damage to the photoconductive antenna due to excessive optical power of the femtosecond laser, precise control of the optical power of the femtosecond laser has become a key part of the terahertz time-domain spectroscopy system.

[0004] At present, mature femtosecond lasers on the market include Langyan lasers, Toptick lasers, etc., and their detection and emission optical powers are fixed, ranging from 10mW to 100mW. However, when femtosecond lasers are applied to terahertz time-domain spectroscopy systems, the optical power of the laser needs to be adjusted to adapt to different fiber lengths and spatial optical paths. Therefore, the application of optical power attenuators becomes inevitable.

[0005] The most widely used one at present is the mechanical variable attenuator (MVOA), which works by expanding the fiber mode field diameter to a few millimeters, and by adjusting the knob, driving the baffle to block the fiber optical path to achieve the attenuation effect. MVOA has very good optical parameters such as less than 1.5dB insertion loss, -30dB return loss, and 0.01dB resolution. Although MVOA has good reliability and high precision, it also has disadvantages such as complex debugging, slow response speed, large size, and difficult to lock after adjustment. From the current research and application situation at home and abroad, the electrically controlled variable attenuator (EVOA) is also in use, with the characteristics of short response time and high integration, but it has disadvantages such as high cost and inability to adjust automatically. Summary of the invention:

[0006] In view of the problems of complex debugging, slow response speed, and difficulty in locking after adjustment of the current MVOA, as well as the high cost of the existing EVOA and the inability to automatically adjust, the present invention discloses a method and device for adaptively adjusting the optical power of a terahertz time-domain spectroscopy system. The main control MCU achieves precise control of the EVOA attenuation value by adjusting the pulse duty cycle, thereby achieving adaptive adjustment of the optical power. The present invention overcomes the above defects of the existing MVOA and EVOA, and has the characteristics of simple debugging, fast response speed, high control accuracy, adaptive adjustment, good real-time performance, etc., and can be widely used in terahertz time-domain spectroscopy systems.

[0007] In order to achieve the above-mentioned object, the present invention relates to an optical power adaptive adjustment device for a terahertz time-domain spectroscopy system, comprising a femtosecond laser, an emission electrically controlled adjustable attenuator, a detection electrically controlled adjustable attenuator, an emission optical splitter, a detection optical splitter, an electrically controlled adjustable attenuator power supply control unit, an optical power conversion and transmission module and a main control MCU;

[0008] The femtosecond laser is used to emit two femtosecond pulses, one of which is a pump light and the other is a detection light. The emission electrically controlled adjustable attenuator is used to receive the pump light. The emission light splitter is connected to the emission electrically controlled adjustable attenuator and is used to split the pump light into a first light beam and a second light beam. The detection electrically controlled adjustable attenuator is used to receive the detection light. The detection light splitter is connected to the detection electrically controlled adjustable attenuator and is used to split the detection light into a third light beam and a fourth light beam.

[0009] Among them, the second light beam is sent to the THZ signal transmitter, the fourth light beam is sent to the THZ signal detector, the first light beam and the third light beam are both sent to the optical power conversion and transmission module, the optical power conversion and transmission module is used to collect, transmit and convert the optical power signals sent by the transmitting optical splitter and the detection optical splitter, the main control MCU is respectively connected to the optical power conversion and transmission module and the electrically controlled adjustable attenuator power control unit, the electrically controlled adjustable attenuator power control unit is respectively connected to the transmitting electrically controlled adjustable attenuator and the detection electrically controlled adjustable attenuator, the main control MCU collects and analyzes the optical power signal sent by the optical power conversion and transmission module, compares it with the corresponding transmitting end optical power threshold or the detection end optical power threshold, and outputs the corresponding pulse duty cycle signal to the electrically controlled adjustable attenuator power control unit, and the electrically controlled adjustable attenuator power control unit outputs voltage to the transmitting electrically controlled adjustable attenuator and the detection electrically controlled adjustable attenuator based on the received pulse duty cycle signal.

[0010] Specifically, the optical power conversion and transmission module includes a photodiode, a converter module and an A / D module. The first light beam and the third light beam are both sent to the photodiode. The photodiode is used to convert the optical signal into a photocurrent signal. The converter module is connected to the photodiode to convert the photocurrent signal into a voltage signal. The A / D module is connected to the converter module to convert the analog voltage signal into a digital voltage signal. The main control MCU is connected to the A / D module.

[0011] The main control MCU is connected to the femtosecond laser to control the opening, closing, resetting, locking and other working processes of the femtosecond laser. The host computer PC is connected to the main control MCU to set the optical power of the femtosecond pulse laser, the optical power threshold of the transmitting end and the optical power threshold of the detecting end.

[0012] Preferably, the splitting ratio of the transmitting light splitter and the detecting light splitter is 5:95.

[0013] To further illustrate, the converter module 8 uses the AD8304ARUZ-RL7 wide dynamic range chip from Analog Devices.

[0014] Further explanation: the power control unit of the electronically controlled adjustable attenuator is designed based on the Rohm BD9227F power chip, with a rated operating voltage of 6V to 20V, a switching frequency of 1MHz, and a maximum output current of 1A.

[0015] The present invention relates to an optical power adaptive adjustment method for a terahertz time-domain spectroscopy system, which specifically comprises the following steps:

[0016] 1) The light beam at the detection end of the femtosecond laser enters the detection electrically controlled adjustable attenuator through the optical fiber, and the attenuated light beam enters the detection optical splitter, which splits one optical fiber into two optical fibers, one of which is connected to the photodiode, and the other is connected to the THZ signal detector. Similarly, the light beam at the output end of the femtosecond laser passes through the emission electrically controlled adjustable attenuator and the emission optical splitter in turn, and the emission optical splitter splits one optical fiber into two optical fibers, one of which is connected to the photodiode, and the other is connected to the THZ signal transmitter;

[0017] 2) The photodiode 7 converts the light signal into a photocurrent, and the photocurrent is converted into a voltage signal by a converter. The converted voltage signal passes through an A / D module and is collected by the main control MCU;

[0018] 3) After the main control MCU analyzes the collected data, it compares the analyzed transmit optical power value with the transmit optical power threshold. If the value is greater than the maximum value, the main control MCU sends a signal to the electronically controlled adjustable attenuator power control unit to increase the duty cycle of the transmit pulse and increase the transmit electronically controlled adjustable attenuator input voltage, thereby reducing the transmit optical power; if the analyzed value is less than the minimum value, the main control MCU sends a signal to the electronically controlled adjustable attenuator power control unit to reduce the duty cycle of the transmit pulse and reduce the transmit electronically controlled adjustable attenuator input voltage, thereby increasing the transmit optical power, and repeats this process until the analyzed value is greater than the minimum value. If the value is within the threshold range, similarly, the optical power value of the detection end after analysis is compared with the optical power threshold of the detection end. If the value is greater than the maximum value, the main control MCU sends a signal to the power control unit of the electrically controlled adjustable attenuator to increase the duty cycle of the detection end pulse, increase the input voltage of the detection electrically controlled adjustable attenuator, and thus reduce the optical power of the detection end; if the value after analysis is less than the minimum value, the main control MCU sends a signal to the power control unit of the electrically controlled adjustable attenuator to reduce the duty cycle of the detection end pulse, reduce the input voltage of the detection electrically controlled adjustable attenuator, and thus increase the emission optical power of the detection end, and repeat this process until the value after analysis is within the threshold range;

[0019] 4) The host computer PC11 analyzes the data uploaded by the main control MCU10 and displays it in real time through the host computer PC11. At the same time, the setting of the laser optical power threshold and the implementation of functional instructions are all implemented in the host computer PC11.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The optical power attenuator used in the existing terahertz time-domain spectroscopy system is MVOA, which is complex to debug, needs to be disassembled for debugging, has a slow response speed, and is not easy to lock after adjustment. The present invention realizes the adaptive adjustment of EVOA through the synergy of the main control MCU and the power control unit of the electronically controlled adjustable attenuator, which is simple to debug, has a fast response speed, high control accuracy and low cost.

[0022] 2. Based on the existing optical splitter, PD and high-performance A / D conversion chip, a method for accurate optical power acquisition at the transmitter and detector of the terahertz time-domain spectroscopy system was proposed for the first time, which realized real-time monitoring and display of optical power at the transmitter and detector, providing closed-loop guarantee for adaptive control of optical power.

[0023] 3. For the first time, an adaptive optical power adjustment method suitable for terahertz time-domain spectroscopy system was proposed, and a set of optical power logic operation methods were designed, which can adaptively adjust the optical power of femtosecond pulses to achieve the optimization of the extreme value difference and dynamic range of terahertz signals.

[0024] 4. The present invention designs for the first time a set of adaptive EVOAs with high control accuracy, good linearity and low cost suitable for terahertz time-domain spectroscopy systems, which can control the linear output of voltage and the linear attenuation of optical power. Description of the drawings:

[0025] Figure 1 Schematic diagram of the structural principle of the optical power adaptive adjustment device used in the terahertz time-domain spectroscopy system.

[0026] Figure 2 It is the current-voltage transfer characteristic curve.

[0027] Figure 3 This is the output voltage linearity curve.

[0028] Figure 4 This is the circuit schematic diagram of BD9227F. Specific implementation method:

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0030] Embodiment 1:

[0031] like Figure 1 As shown, the present embodiment involves an optical power adaptive adjustment device for a terahertz time-domain spectroscopy system, comprising a femtosecond laser 1, an emission electrically-controlled adjustable attenuator 2, a detection electrically-controlled adjustable attenuator 3, an emission optical splitter 4, a detection optical splitter 5, an electrically-controlled adjustable attenuator power control unit 6, a photodiode 7, a converter module 8, an A / D module 9, a main control MCU 10 and a host computer PC 11.

[0032] The femtosecond laser 1 is used to emit two femtosecond pulses, one of which is a pump light and the other is a detection light. The emission electrically controlled variable attenuator 2 is used to receive the pump light, and the emission light splitter 4 is connected to the emission electrically controlled variable attenuator 2 to split the pump light into a first light beam and a second light beam. The detection electrically controlled variable attenuator 3 is used to receive the detection light, and the detection light splitter 5 is connected to the detection electrically controlled variable attenuator 3 to split the detection light into a third light beam and a fourth light beam.

[0033] Among them, the second light beam is sent to the THZ signal transmitter (transmitting end), the fourth light beam is sent to the THZ signal detector (detecting end), the first light beam and the third light beam are both sent to the photodiode 7, the photodiode 7 is used to convert the optical signal into a photocurrent signal, the converter module 8 is connected to the photodiode 7 for converting the photocurrent signal into a voltage signal, the A / D module 9 is connected to the converter module 8 for converting the analog voltage signal into a digital voltage signal, the main control MCU 10 is respectively connected to the A / D module 9 and the electrically controlled adjustable attenuator power supply control unit 6, the electrically controlled adjustable attenuator power supply control unit 6 is respectively connected to the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3, the main control MCU 10 collects and analyzes the digital voltage signal sent by the A / D module 9, compares it with the corresponding transmitting end optical power threshold or the detecting end optical power threshold, and outputs the corresponding pulse duty cycle signal to the electrically controlled adjustable attenuator power supply control unit 6, and the electrically controlled adjustable attenuator power supply control unit 6 outputs voltage to the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3 based on the received pulse duty cycle signal.

[0034] The main control MCU 10 is connected to the femtosecond laser 1 and is used to control the opening, closing, resetting, locking and other working processes of the femtosecond laser 1. The host computer PC 11 is connected to the main control MCU 10 and is used to set the optical power of the femtosecond pulse laser, the optical power threshold of the transmitting end and the optical power threshold of the detecting end.

[0035] Preferably, the splitting ratio of the emission optical splitter 4 and the detection optical splitter 5 is 5:95, that is, the optical power of the first light beam is 5% of the pump light, the optical power of the second light beam is 95% of the pump light, the optical power of the third light beam is 5% of the detection light, and the optical power of the fourth light beam is 95% of the detection light. In the specific implementation process, optical splitters with other splitting ratios can also be selected as needed.

[0036] Further explanation: In order to achieve precise control of the optical power of the femtosecond laser, precise detection of the optical power at the transmitting end and the detecting end is particularly important. Our converter module 8 uses the AD8304ARUZ-RL7 wide dynamic range chip of ADI Company, which realizes the conversion of weak photocurrent to voltage. The current detection range is as high as 100pA~10mA, and the dynamic range can reach 160dB. At the same time, the adaptive bias technology of AD8304ARUZ-RL7 can directly dynamically bias the photodiode 7 to reduce the influence of the dark current of the photodiode 7. When the photocurrent is 100pA, the voltage is 0.1V, and when the photocurrent is 10mA, the voltage is 2V. The voltage changes linearly with the photocurrent, which is 237.5mV / dec. The conversion slope can be configured through an external circuit, such as Figure 2 The current-voltage transfer characteristic curve is shown in FIG.

[0037] To further explain, the A / D module 9 uses the 16-bit AD677JNZ analog-to-digital converter of ADI Company. The chip has an automatic calibration function and a conversion rate of up to 100kSPS. At the same time, the analog buffer provides high input impedance, reducing the impact on the previous stage.

[0038] It is further explained that the optical power threshold of the transmitting end and the optical power threshold of the detecting end are set based on the extreme value and dynamic range of the terahertz time-domain spectroscopy system.

[0039] It is further explained that the attenuation range of the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3 is adjusted according to the change of the input voltage. The greater the input voltage, the greater the attenuation and the smaller the output optical power. Specifically, the attenuation range of the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3 is 0-40dB, the insertion loss is 0.55dB, the response time is ≤5ms, the transmitted optical power is 300mw, the voltage is 4.3V when the attenuation value is 40dB, the accuracy is ≤0.05dB, and the maximum working voltage is 6V. The corresponding relationship between the attenuated optical power and the input voltage is shown in Table 1.

[0040] Table 1 Correspondence between input voltage and attenuated output power

[0041] Input voltage(V) Output power (mW) 0 50 0.5 45.73 1 40.68 1.5 35.07 2 29.98 2.5 25.32 3 20.15 3.5 15.76 4 10.28 4.5 5.01 5 1.06

[0042] Further explanation, the power control unit 6 of the electrically controlled adjustable attenuator is designed based on the Rohm BD9227F power chip, with a rated working voltage of 6V to 20V, a switching frequency of 1MHz, and a maximum output current of 1A. The power chip controls its own output voltage according to the duty cycle of the pulse signal provided by the main control MCU10. The control circuit has the characteristics of high linearity, high control accuracy, simple circuit, and high power conversion efficiency. The output voltage changes linearly with the pulse duty cycle (PWM duty), such as Figure 3 shown.

[0043] Further explanation: BD9227F controls the output voltage range of 0-5V, which meets the attenuation range requirements and use requirements of the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3. The control accuracy of BD9227F is as high as 0.003mV, and the corresponding control accuracy of EVOA is about 0.024dB. Its control meets the accuracy requirements of the transmitting electrically controlled adjustable attenuator 2 and the detecting electrically controlled adjustable attenuator 3, thereby achieving linear adjustment.

[0044] The present embodiment relates to an optical power adaptive adjustment method for a terahertz time-domain spectroscopy system, which specifically includes the following steps:

[0045] 5) The light beam at the detection end of the femtosecond laser 1 enters the detection electrically controlled adjustable attenuator 3 through the optical fiber, and the attenuated light beam enters the detection optical splitter 5. The detection optical splitter 5 divides one optical fiber into two optical fibers, one of which is connected to the photodiode 7, and the other is connected to the THZ signal detector. Similarly, the light beam at the output end of the femtosecond laser 1 passes through the emission electrically controlled adjustable attenuator 2 and the emission optical splitter 4 in sequence. The emission optical splitter 4 divides one optical fiber into two optical fibers, one of which is connected to the photodiode 7, and the other is connected to the THZ signal transmitter. Preferably, the splitting ratios of the detection optical splitter 5 and the emission optical splitter 4 are both 5:95, that is, 5% of the optical power is taken out and passed through the photodiode, and 95% of the optical power is transmitted to the THZ signal detector or THZ signal transmitter through the optical fiber.

[0046] 6) The photodiode 7 converts 5% of the light signal into a photocurrent, and the photocurrent is converted into a voltage signal by the converter module 8. The converted voltage signal passes through the A / D module 9 and is collected by the main control MCU 10.

[0047] 7) After the main control MCU 10 parses the collected data, it compares the analyzed transmit optical power value with the transmit optical power threshold (the threshold includes the maximum value and the minimum value). If the value is greater than the maximum value, the main control MCU 10 sends a signal to the electrically controlled adjustable attenuator power supply control unit 6 to increase the duty cycle of the transmit pulse, increase the transmit electrically controlled adjustable attenuator 2 input voltage, and then increase the attenuation value of the transmit electrically controlled adjustable attenuator 2, and reduce the optical power of the transmit end; if the analyzed value is less than the minimum value, the main control MCU 10 sends a signal to the electrically controlled adjustable attenuator power supply control unit 6 to reduce the duty cycle of the transmit pulse, reduce the transmit electrically controlled adjustable attenuator 2 input voltage, and then reduce the attenuation value of the transmit electrically controlled adjustable attenuator 2, and increase the transmit optical power of the transmit end, and repeat this process until the analysis is completed. The value is within the threshold range. Similarly, the optical power value of the detection end after analysis is compared with the optical power threshold of the detection end (the threshold includes the maximum value and the minimum value). If the value is greater than the maximum value, the main control MCU10 sends a signal to the power control unit 6 of the electrically controlled adjustable attenuator to increase the duty cycle of the detection end pulse, increase the input voltage of the detection electrically controlled adjustable attenuator 3, and then increase the attenuation value of the detection electrically controlled adjustable attenuator 3, and reduce the optical power of the detection end; if the value after analysis is less than the minimum value, the main control MCU10 sends a signal to the power control unit 6 of the electrically controlled adjustable attenuator to reduce the duty cycle of the detection end pulse, reduce the input voltage of the detection electrically controlled adjustable attenuator 3, and then reduce the attenuation value of the detection electrically controlled adjustable attenuator 3, and increase the emission optical power of the detection end, and repeat until the value after analysis is within the threshold range. At the same time, the main control MCU10 needs to communicate with the host computer PC11 and realize the control of each component.

[0048] 8) The host computer PC11 analyzes the data uploaded by the main control MCU10 and displays it in real time through the host computer PC11. At the same time, the setting of the laser optical power threshold and the implementation of functional instructions are all implemented in the host computer PC11.

Claims

1. An optical power adaptive adjustment device for a terahertz time-domain spectroscopy system, characterized in that: Including femtosecond laser, transmitting electrically controlled adjustable attenuator, detecting electrically controlled adjustable attenuator, transmitting optical splitter, detecting optical splitter, electrically controlled adjustable attenuator power control unit, optical power conversion and transmission module, and main control MCU; The femtosecond laser is used to emit two femtosecond pulses, one of which is a pump light and the other is a detection light. The emission electrically controlled adjustable attenuator is used to receive the pump light. The emission light splitter is connected to the emission electrically controlled adjustable attenuator and is used to split the pump light into a first light beam and a second light beam. The detection electrically controlled adjustable attenuator is used to receive the detection light. The detection light splitter is connected to the detection electrically controlled adjustable attenuator and is used to split the detection light into a third light beam and a fourth light beam. Among them, the second light beam is sent to the THZ signal transmitter, the fourth light beam is sent to the THZ signal detector, the first light beam and the third light beam are both sent to the optical power conversion and transmission module, the optical power conversion and transmission module is used to collect, transmit and convert the optical power signals sent by the transmitting optical splitter and the detection optical splitter, the main control MCU is respectively connected to the optical power conversion and transmission module and the electrically controlled adjustable attenuator power control unit, the electrically controlled adjustable attenuator power control unit is respectively connected to the transmitting electrically controlled adjustable attenuator and the detection electrically controlled adjustable attenuator, the main control MCU collects and analyzes the optical power signal sent by the optical power conversion and transmission module, compares it with the corresponding transmitting end optical power threshold or the detection end optical power threshold, and outputs the corresponding pulse duty cycle signal to the electrically controlled adjustable attenuator power control unit, and the electrically controlled adjustable attenuator power control unit outputs voltage to the transmitting electrically controlled adjustable attenuator and the detection electrically controlled adjustable attenuator based on the received pulse duty cycle signal.

2. The optical power adaptive adjustment device for a terahertz time-domain spectroscopy system according to claim 1, characterized in that: The optical power conversion and transmission module includes a photodiode, a converter module and an A / D module. The first light beam and the third light beam are both sent to the photodiode. The photodiode is used to convert the optical signal into a photocurrent signal. The converter module is connected to the photodiode to convert the photocurrent signal into a voltage signal. The A / D module is connected to the converter module to convert the analog voltage signal into a digital voltage signal. The main control MCU is connected to the A / D module.

3. The optical power adaptive adjustment device for a terahertz time-domain spectroscopy system according to claim 1, characterized in that: The main control MCU is connected to the femtosecond laser to control the opening, closing, resetting and locking of the femtosecond laser. The host computer PC is connected to the main control MCU to set the optical power of the femtosecond pulse laser, the optical power threshold of the transmitting end and the optical power threshold of the detecting end.

4. The optical power adaptive adjustment device for a terahertz time-domain spectroscopy system according to claim 1, characterized in that: The splitting ratio of the optical splitter is 5:

95.

5. The optical power adaptive adjustment device for a terahertz time-domain spectroscopy system according to claim 1, characterized in that: The converter module uses the AD8304ARUZ-RL7 wide dynamic range chip from ADI; the electronically controlled adjustable attenuator power control unit is designed based on the Rohm BD9227F power chip, with a rated operating voltage of 6V to 20V, a switching frequency of 1MHz, and a maximum output current of 1A.

6. A method for adaptively adjusting optical power in a terahertz time-domain spectroscopy system, characterized in that: The specific steps include: 1) The light beam at the detection end of the femtosecond laser enters the detection electrically controlled adjustable attenuator through the optical fiber, and the attenuated light beam enters the detection optical splitter, which splits one optical fiber into two optical fibers, one of which is connected to the photodiode, and the other is connected to the THZ signal detector. Similarly, the light beam at the output end of the femtosecond laser passes through the emission electrically controlled adjustable attenuator and the emission optical splitter in turn, and the emission optical splitter splits one optical fiber into two optical fibers, one of which is connected to the photodiode, and the other is connected to the THZ signal emission; 2) The photodiode converts the light signal into a photocurrent, and the photocurrent is converted into a voltage signal by a converter. The converted voltage signal passes through the A / D module and is collected by the main control MCU; 3) After the main control MCU analyzes the collected data, it compares the analyzed transmit optical power value with the transmit optical power threshold. If the value is greater than the maximum value, the main control MCU sends a signal to the electronically controlled adjustable attenuator power control unit to increase the duty cycle of the transmit pulse and increase the transmit electronically controlled adjustable attenuator input voltage, thereby reducing the transmit optical power; if the analyzed value is less than the minimum value, the main control MCU sends a signal to the electronically controlled adjustable attenuator power control unit to reduce the duty cycle of the transmit pulse and reduce the transmit electronically controlled adjustable attenuator input voltage, thereby increasing the transmit optical power, and repeats this process until the analyzed value is greater than the minimum value. If the value is within the threshold range, the optical power value of the detection end after analysis is compared with the optical power threshold of the detection end. If the value is greater than the maximum value, the main control MCU sends a signal to the power control unit of the electrically controlled adjustable attenuator to increase the duty cycle of the detection end pulse, increase the input voltage of the detection electrically controlled adjustable attenuator, and thereby reduce the optical power of the detection end; if the value after analysis is less than the minimum value, the main control MCU sends a signal to the power control unit of the electrically controlled adjustable attenuator to reduce the duty cycle of the detection end pulse, reduce the input voltage of the detection electrically controlled adjustable attenuator, and thereby increase the emission optical power of the detection end, and repeat this process until the value after analysis is within the threshold range.

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