A method and device for detecting fA-level weak signals in terahertz spectroscopy and imaging systems

Abstract

The present invention discloses a fA-level weak signal detection device and method for a terahertz spectroscopy and imaging system. A TIA pre-current amplifier is connected to a detection antenna of a current-type terahertz spectroscopy and imaging system, a pre-gain amplifier is connected to a TIA pre-current amplifier, or is connected to a detection antenna of a voltage-type terahertz spectroscopy and imaging system; a BPF bandpass filter input end is connected to a pre-gain amplifier; a bandpass gain amplifier is connected to a BPF bandpass filter; a MODEM modem is respectively connected to a bandpass gain amplifier and a main control MCU; an LPF low-pass filter is connected to a MODEM modem; the low-pass gain amplifier is connected to an LPF low-pass filter; an AD analog-to-digital converter is connected to a low-pass gain amplifier; and a main control MCU is respectively connected to a MODEM modem, an AD analog-to-digital converter, and a PC host computer. The acquisition accuracy and acquisition range thereof are greatly improved compared to existing weak signal acquisition devices, and fA-level current signal acquisition can be realized, and the acquisition gain is 1*10^ 4 to 1*10^ 9 Precisely adjustable.

Description

Technical field:

[0001] The present invention belongs to the technical field of terahertz spectroscopy and imaging technology, and specifically relates to a fA-level weak signal detection method and device for a terahertz spectroscopy and imaging system. Background technology:

[0002] Terahertz waves are electromagnetic waves with strong penetrating power and low photon energy, with a frequency of 0.1 to 10 THz (1 THz = 10 12 Hz), between microwaves and infrared waves. Terahertz spectroscopy and imaging systems use terahertz waves for photoconductive sampling and are widely used in material identification, security 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, which form current under the action of the terahertz pulse electric field. The terahertz pulse can be measured by detecting the change in current. Affected by environmental radiation and the internal noise of the detection equipment, the detection current signal is very small, even submerged by the noise, and belongs to the nA level current signal.

[0004] In order to improve the signal's anti-interference ability and achieve accurate detection, we need to extract, recover and enhance the measured signal from the noise. Therefore, accurate detection of this weak signal has become a key part of the terahertz spectroscopy and imaging technology field.

[0005] Bandpass filter is a common method for weak signal detection. It compresses the bandwidth of the detection channel so that it can cover the spectrum of the signal. The noise output will be significantly reduced, so it is the most common method to reduce noise and improve the signal-to-noise ratio. However, due to the limitation of its frequency stability, simply using compressed bandwidth to suppress noise is limited. Summary of the invention:

[0006] The purpose of the present invention is to seek a fA-level weak signal detection method and device for terahertz spectroscopy and imaging systems.

[0007] In order to achieve the above-mentioned purpose, the present invention relates to a fA-level weak signal detection device for a terahertz spectroscopy and imaging system, comprising a TIA pre-current amplifier, a pre-gain amplifier, a BPF band-pass filter, a band-pass gain amplifier, a MODEM modem, an LPF low-pass filter, a low-pass gain amplifier, an AD analog-to-digital converter, a POWER power supply module, a main control MCU and a PC host computer; the TIA pre-current amplifier 1 input end is connected to the detection antenna of the current-type terahertz spectroscopy and imaging system, and is used to collect and amplify the bias current signal generated by the detection antenna; the pre-gain amplifier input end is connected to the TIA pre-current amplifier output end, or is connected to the detection antenna of the voltage-type terahertz spectroscopy and imaging system, and is used to further amplify the input signal; the BPF band-pass filter input end is connected to the pre-gain amplifier output end, and is used to filter out high-frequency and low-frequency noise; the band-pass gain amplifier input end is connected to the BPF band-pass filter output end, and is used to further amplify the input signal; the MODEM modem The input end of the modem is connected to the output end of the bandpass gain amplifier and the main control MCU respectively, and is used to multiply the reference signal input by the main control MCU with the signal input by the bandpass gain amplifier, and output a DC signal, and the reference signal is in the same frequency as the input signal of the detection antenna; the input end of the LPF low-pass filter is connected to the output end of the MODEM modem, and is used to filter out the AC noise signal and output a DC useful signal; the input end of the low-pass gain amplifier is connected to the output end of the LPF low-pass filter, and is used to further amplify the signal; the input end of the AD analog-to-digital converter is connected to the output end of the low-pass gain amplifier, and is used to convert the analog signal into a digital signal, and transmit the digital signal to the main control MCU; the main control MCU is connected to the MODEM modem, the AD analog-to-digital converter and the PC host computer respectively, and is used to output a reference signal in the same frequency as the bias current signal, receive the digital signal sent by the AD analog-to-digital converter and upload it to the PC host computer, and the PC host computer performs data processing and presents a terahertz spectrum in the form of time domain waveform and frequency domain waveform.

[0008] It is further explained that the fA-level weak signal detection device for terahertz spectroscopy and imaging system also includes a shell, and a TIA pre-current amplifier, a pre-gain amplifier, a BPF bandpass filter, a bandpass gain amplifier, a MODEM modem, an LPF low-pass filter, a low-pass gain amplifier, an AD analog-to-digital converter, a POWER power supply module and a main control MCU are all built into the shell.

[0009] To further illustrate, the POWER power supply module is respectively connected to the TIA pre-current amplifier, the pre-gain amplifier, the BPF band-pass filter, the band-pass gain amplifier, the MODEM modem, the LPF low-pass filter, the low-pass gain amplifier, the AD analog-to-digital converter and the main control MCU.

[0010] Further explanation: the TIA pre-current amplifier part uses a fA-class op amp, whose input bias current can be as low as 20fA, offset voltage can be as low as 50uV, has low offset voltage drift, and gain can be adjusted to 1*10^ 4 to 1*10^ 9 , the voltage noise density can be as low as 14nV / √Hz (10KHz) and the bandwidth can reach 2MHz.

[0011] Furthermore, the pre-gain amplifier has an amplification factor of 1, 2, 5 or 10 times, the band-pass gain amplifier has an amplification factor of 1, 2, 5 or 10 times, the low-pass gain amplifier has an amplification factor of 2, 5, 10 or 20 times, and the LPF low-pass filter cutoff frequency can be selected from 3 Hz, 10 Hz, 30 Hz or 100 Hz.

[0012] As an implementation method, the PC host computer includes a phase adjustment module, a three-level gain adjustment module, a low-pass cutoff frequency adjustment module and a display module. The phase adjustment module is used to adjust the phase of the reference signal, and the phase adjustment range is 0-360°; the three-level gain adjustment module is used to adjust the amplification factors of the pre-gain amplifier, the band-pass gain amplifier and the low-pass gain amplifier; the low-pass cutoff frequency adjustment module is used to adjust the cutoff frequency of the LPF low-pass filter, the pre-gain amplifier, the band-pass gain amplifier, the low-pass gain amplifier and the LPF low-pass filter are all connected to the main control MCU, and the display module is used to input and display the adjusted phase degree, gain multiple and frequency, and display the terahertz spectrum in real time.

[0013] As another implementation method, a dip switch and a rotary encoder are set on the shell panel, and the dip switch is respectively connected to the pre-gain amplifier, the bandpass gain amplifier, the low-pass gain amplifier and the LPF low-pass filter, and is used for adjusting the gain of the pre-gain amplifier, the bandpass gain amplifier and the low-pass gain amplifier and the cutoff frequency of the LPF low-pass filter. The rotary encoder is connected to the main control MCU and is used to input the adjusted phase value into the main control MCU, and the main control MCU outputs a reference signal for phase adjustment.

[0014] Further explanation: a voltage collection input port VIN and a current collection input port CIN are provided on the shell panel, wherein the voltage collection input port VIN is internally connected to a TIA pre-current amplifier and an external detection antenna for outputting current signals, and the current collection input port CIN is internally connected to a pre-gain amplifier and an external detection antenna for outputting voltage signals.

[0015] An analog voltage output port OUT is provided on the shell panel, and the analog voltage output port OUT is internally connected to the output end of the low-pass gain amplifier. A modulation signal output port CLK is provided on the shell panel, and the modulation signal output port CLK is internally connected to the reference signal output end of the main control MCU. A USB interface is provided on the shell panel, and the main control MCU is connected to the PC host computer through the USB interface. The shell panel is provided with an OFF / ON button for starting and shutting down the device, a power input port (24V) and fault, power and status indicator lights.

[0016] The implementation process of the fA-level weak signal detection method for terahertz spectroscopy and imaging system involved in the present invention is as follows:

[0017] 1) The current signal received by the detection antenna is first amplified by the TIA pre-current amplifier;

[0018] 2) After passing through the TIA pre-current amplifier 1, the signal enters the pre-gain amplifier for further amplification;

[0019] 3) After the pre-gain amplifier signal is output, it enters the BPF bandpass filter to filter out high-frequency and low-frequency noise;

[0020] 4) After the BPF bandpass filter output, the signal enters the bandpass gain amplifier for further amplification;

[0021] 5) The phase of the reference signal output by the main control MCU is adjusted through a PC host computer or a rotary encoder. The phase adjustment range is 0-360°. The reference signal is at the same frequency as the input signal output by the detection antenna.

[0022] 6) After passing through the bandpass gain amplifier, the signal enters the MODEM modem. At this time, the main control MCU provides a reference signal to perform multiplication operation with the input signal;

[0023] 7) The output DC signal enters the LPF low-pass filter to filter out the AC noise signal and output a DC useful signal;

[0024] 8) The signal after passing through the LPF low-pass filter enters the low-pass gain amplifier for further amplification;

[0025] 9) After low-pass gain, the signal passes through an AD analog-to-digital converter to convert the analog signal into a digital signal. After the main control MCU 10 collects the digital signal, the data is uploaded to the PC host computer 11. After the PC host computer 11 processes the data, it is presented in the form of a time domain waveform and a frequency domain waveform;

[0026] Repeat steps 5)-9) until the extreme value difference of the spectrum output by the PC host computer 11 is the largest, and the adjustment is completed.

[0027] Compared with the prior art, the present invention has the following beneficial effects: the acquisition accuracy and acquisition range are greatly improved compared with the existing weak signal acquisition device, the acquisition range can achieve fA level current signal acquisition, and the acquisition gain can achieve 1*10^ 4 to 1*10^ 9 Precise and adjustable; two independent acquisition ports can realize modulation and demodulation of current and voltage signals respectively, with stronger compatibility; one modulation signal output and one analog signal output, which is easy to detect and debug; the output signal gain is adjustable in three levels, which can adapt to the signal acquisition and conversion of different AD analog-to-digital conversion chips; USB virtual serial port connects to the host computer, and the minimalist phase-locked control interface realizes function control and parameter setting while realizing accurate analysis and real-time presentation of the collected signal; the reference signal is internally synchronized, and no external reference signal input is required, and the frequency can be flexibly set according to user needs; phase adjustment can meet two adjustment methods of host computer and rotary encoder, and the adjustable range is 0°-360°; the low-pass filter cutoff frequency can be selected at 3Hz, 10Hz, 30Hz, and 100Hz. Description of the drawings:

[0028] Figure 1 The present invention is a schematic structural diagram of a fA-level weak signal detection device for a terahertz spectroscopy and imaging system.

[0029] Figure 2 The present invention is a schematic diagram of a PC host computer display module.

[0030] Figure 3 It is a schematic diagram of a housing panel according to the present invention. Specific implementation method:

[0031] 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.

[0032] Embodiment 1:

[0033] like Figure 1 As shown: This embodiment involves a fA-level weak signal detection device for a terahertz spectroscopy and imaging system, including a TIA pre-current amplifier 1, a pre-gain amplifier 2, a BPF band-pass filter 3, a band-pass gain amplifier 4, a MODEM modem 5, an LPF low-pass filter 6, a low-pass gain amplifier 7, an AD analog-to-digital converter 8, a POWER power supply module 9, a main control MCU 10 and a PC host computer 11.

[0034] The input end of the TIA pre-current amplifier 1 is connected to the detection antenna of the current-type terahertz spectroscopy and imaging system, and is used to collect and amplify the bias current signal generated by the detection antenna;

[0035] The input end of the pre-gain amplifier 2 is connected to the output end of the TIA pre-current amplifier 1, or is connected to the detection antenna of the voltage-type terahertz spectroscopy and imaging system, and is used to further amplify the input signal. The amplification factor can be selected to be 1 times, 2 times, 5 times, or 10 times;

[0036] The input end of the BPF bandpass filter 3 is connected to the output end of the pre-gain amplifier 2, and is used to filter out high-frequency and low-frequency noise, and its center frequency is adjustable;

[0037] The input end of the bandpass gain amplifier 4 is connected to the output end of the BPF bandpass filter 3, and is used to further amplify the input signal. The amplification factor can be selected to be 1 times, 2 times, 5 times, or 10 times;

[0038] The input end of the MODEM modem 5 is connected to the output end of the bandpass gain amplifier 4 and the main control MCU 10 respectively, and is used to multiply the reference signal input by the main control MCU 10 with the signal input by the bandpass gain amplifier 4, and output a DC signal, wherein the reference signal has the same frequency as the input signal of the detection antenna;

[0039] The input end of the LPF low-pass filter 6 is connected to the output end of the MODEM modem 5, and is used to filter out AC noise signals and output DC useful signals. The cut-off frequency is adjustable and can be selected from 3Hz, 10Hz, 30Hz, and 100Hz;

[0040] The input end of the low-pass gain amplifier 7 is connected to the output end of the LPF low-pass filter 6 to further amplify the signal. The amplification factor can be selected to be 2 times, 5 times, 10 times, or 20 times;

[0041] The input end of the AD analog-to-digital converter 8 is connected to the output end of the low-pass gain amplifier 7, and is used to convert the analog signal into a digital signal, and transmit the digital signal to the main control MCU 10;

[0042] The main control MCU 10 is connected to the TIA pre-current amplifier 1, the MODEM modem 5, the AD analog-to-digital converter 8 and the PC host computer 11 respectively, and is used to output a REF_CLK reference signal with the same frequency as the bias current signal, receive the digital signal sent by the AD analog-to-digital converter 8 and upload it to the PC host computer 11. The PC host computer 11 performs data processing and presents a terahertz spectrum in the form of a time domain waveform and a frequency domain waveform.

[0043] It is further explained that the fA-level weak signal detection device for terahertz spectroscopy and imaging system also includes a shell, and a TIA pre-current amplifier 1, a pre-gain amplifier 2, a BPF bandpass filter 3, a bandpass gain amplifier 4, a MODEM modem 5, an LPF low-pass filter 6, a low-pass gain amplifier 7, an AD analog-to-digital converter 8, a POWER power supply module 9 and a main control MCU 10 are all built in the shell.

[0044] Further explanation, the POWER power supply module 9 is respectively connected with the TIA pre-current amplifier 1, the pre-gain amplifier 2, the BPF bandpass filter 3, the bandpass gain amplifier 4, the MODEM modem 5, the LPF low-pass filter 6, the low-pass gain amplifier 7, the AD analog-to-digital converter 8 and the main control MCU 10, and provides the required working voltage of each component as needed. The POWER power supply module 9 uses an isolated power supply and a low-noise LDO, which outputs +5V and -5V respectively, and the power supply ripple is controlled within 10mV.

[0045] Further explanation: the TIA pre-current amplifier 1 part uses fA-class op amps, whose input bias current can be as low as 20fA, offset voltage can be as low as 50uV, with low offset voltage drift, and gain adjustable to 1*10^ 4 to 1*10^ 9 , the voltage noise density can be as low as 14nV / √Hz (10KHz) and the bandwidth can reach 2MHz.

[0046] like Figure 2 As shown, as an implementation method, the reference signal phase adjustment, the gain adjustment of the pre-gain amplifier 2, the band-pass gain amplifier 4 and the low-pass gain amplifier 7 and the cut-off frequency adjustment of the LPF low-pass filter 6 are realized by the corresponding software in the PC host computer 11. Specifically, the PC host computer 11 includes a phase adjustment module, a three-level gain adjustment module and a low-pass cut-off frequency adjustment module, the phase adjustment module is used to adjust the phase of the reference signal, and the phase adjustment range is 0-360°; the three-level gain adjustment module is used to adjust the amplification factor of the pre-gain amplifier 2, the band-pass gain amplifier 4 and the low-pass gain amplifier 7; the low-pass cut-off frequency adjustment module is used to adjust the cut-off frequency of the LPF low-pass filter 6, the pre-gain amplifier 2, the band-pass gain amplifier 4, the low-pass gain amplifier 7 and the LPF low-pass filter 6 are all connected to the main control MCU 10, and the PC host computer 11 sends the adjustment signal to the corresponding components through the main control MCU 10. In addition, the PC host computer 11 includes a display module for inputting and displaying the adjusted phase degree, gain multiple and frequency, and displaying the terahertz spectrum in real time.

[0047] To further explain, when the phase is adjusted in the host computer 11: enter the phase value (0-360°) in the phase edit box, click the "Set" button, or adjust the phase setting knob to set the phase.

[0048] To further explain, the gain of each level is adjusted in the host computer 11: select the corresponding multiples in the pre-gain, band-pass gain, and low-pass gain drop-down boxes respectively, and click the corresponding "Set" to set the amplification multiples respectively.

[0049] To further explain, the low-pass cut-off frequency is adjusted in the host computer 11: select the corresponding frequency in the low-pass cut-off frequency drop-down box, and click the "Set" button to set it.

[0050] To further illustrate, real-time data collection is performed in the host computer 11: the set time interval is filled in the interval text box, and the estimated number of collections is filled in the number of times text box, and "Start Collection" is clicked to collect and display data in real time.

[0051] Further explanation, such as Figure 3 The housing panel is provided with a voltage collection input port VIN and a current collection input port CIN, wherein the voltage collection input port VIN is internally connected to the TIA pre-current amplifier 1 and an external detection antenna for outputting current signals, and the current collection input port CIN is internally connected to the pre-gain amplifier 2 and an external detection antenna for outputting voltage signals. The two collection ports respectively realize the modulation and demodulation of current and voltage signals, and have stronger compatibility. An analog voltage output port OUT is provided on the housing panel, and the analog voltage output port OUT is internally connected to the output end of the low-pass gain amplifier 7. A modulation signal output port CLK is provided on the housing panel, and the modulation signal output port CLK is internally connected to the reference signal output end of the main control MCU10. One CLK modulation signal output with adjustable modulation frequency and one OUT analog signal output are convenient for detecting the amplification factor and debugging.

[0052] The housing panel is provided with a USB interface, through which the main control MCU 10 is connected to the PC host computer 11 to realize communication, data collection, parameter setting, board upgrade and other functions. The housing panel is provided with an OFF / ON button for starting and shutting down the device, a power input port (24V) and fault, power and status indicator lights.

[0053] As another implementation method, a dial switch and a rotary encoder are set on the shell panel to achieve reference signal phase adjustment, pre-gain amplifier 2, band-pass gain amplifier 4 and low-pass gain amplifier 7 gain adjustment and LPF low-pass filter 6 cut-off frequency adjustment. Specifically, a dial switch and a rotary encoder are set on the shell panel, and the dial switch is connected to the pre-gain amplifier 2, band-pass gain amplifier 4, low-pass gain amplifier 7 and LPF low-pass filter 6 respectively, and is used for pre-gain amplifier 2, band-pass gain amplifier 4 and low-pass gain amplifier 7 gain adjustment and LPF low-pass filter 6 cut-off frequency adjustment, and the rotary encoder is connected to the main control MCU 10, and the adjusted phase value is input to the main control MCU 10, and the main control MCU 10 outputs the reference signal for phase adjustment. More specifically, the dial switch includes S1-S12, and each function is shown in Table 2. The rotary encoder includes a coarse adjustment knob and a fine adjustment knob, as shown in Table 1, which can realize the functions of hardware and host computer control software enabling, three-stage amplification adjustment, low-pass filter cut-off frequency selection, modulation signal phase adjustment, etc. The enable control and three-level gain adjustment are detailed in Table 1, and the modulation signal phase adjustment is detailed in Table 2.

[0054] Table 1

[0055]

[0056] Table 2

[0057]

[0058]

[0059] The specific implementation process of the fA-level weak signal detection method for terahertz spectroscopy and imaging system of the present invention is as follows:

[0060] 1) The nA level current signal received by the detection antenna is first amplified by the TIA pre-current amplifier 1, and the amplification gain is 10^ 6 , select a fA-class electrometer amplifier with a minimum input bias current of 20fA.

[0061] 2) The signal amplified by the TIA pre-current amplifier 1 enters the pre-gain amplifier 2. The pre-gain amplifier 2 op amp is set with 4 levels of gain, namely 1x, 2x, 5x, and 10x. The gain can be set according to actual needs through the host computer software and the dial switch.

[0062] 3) After the signal of the pre-gain amplifier 2 is output, it enters the BPF band-pass filter 3. The BPF band-pass filter 3 uses a second-order Butterworth filter, and its center frequency can be selected from 1K, 10K, and 100K according to needs.

[0063] 4) After being output by the BPF bandpass filter 3, the signal enters the bandpass gain amplifier 4. The bandpass gain amplifier 4 op amp is set with 4 levels of amplification factors, namely 1 times, 2 times, 5 times, and 10 times. The amplification factor can be set according to actual needs through the host computer software and the dial switch.

[0064] 5) The phase of the REF_CLK reference signal output by the main control MCU10 is adjusted through a PC host computer or a rotary encoder. The phase adjustment range is 0-360°. The REF_CLK reference signal has the same frequency as the input signal output by the detection antenna.

[0065] 6) After passing through the bandpass gain amplifier 4, the signal enters the MODEM modem 5. At this time, the main control MCU 10 provides a REF_CLK reference signal to perform multiplication operation with the input signal;

[0066] For example, the input signal i 1 =Asin(ωt)+N(t), A is the amplitude, ω is the frequency, ωt is the phase, N(t) is the random noise, and the reference signal i 2 =Bsin(ωt+θ), where B is the amplitude and θ is the phase difference between the two signals. Directly multiply the input signal and the reference signal:

[0067] i 1 *i 2 =[Asin(ωt)+N(t)]*[Bsin(ωt+θ)]=AB / 2cos(θ)-AB / 2cos(2ωt+θ)+BN(t)sin(ωt+θ)

[0068] Among them, AB / 2cos(θ) is the DC component, AB / 2cos(2ωt+θ) is the AC component, the reference signal is uncorrelated with the noise signal, and the DC component of BN(t)sin(ωt+θ) can be regarded as zero. The purpose of phase adjustment is to adjust the θ value to 0, at which time the value of cos(θ) is 1, and the DC component signal strength is the largest, which is convenient for signal acquisition.

[0069] 7) The output DC signal enters the LPF low-pass filter 6. The LPF low-pass filter 6 uses a second-order Butterworth filter, and its cut-off frequency can be selected from 3K, 10K, 30K, and 100K according to requirements.

[0070] 8) The signal after passing through the LPF low-pass filter 6 enters the low-pass gain amplifier 7. The low-pass gain amplifier 7 op amp is set with 4 levels of amplification, which are 2 times, 5 times, 10 times, and 20 times respectively. The amplification can be set according to actual needs through the host computer software and the dial switch.

[0071] 9) After low-pass gain, the signal passes through an AD analog-to-digital converter to convert the analog signal into a digital signal. After the main control MCU 10 collects the digital signal, the data is uploaded to the PC host computer 11. After the PC host computer 11 processes the data, it is presented in the form of a time domain waveform and a frequency domain waveform;

[0072] 10) Repeat steps 5)-9) until the extreme value difference (difference between the maximum value and the minimum value) of the spectrum output by the PC host computer 11 is the largest, and the adjustment is completed.

Claims

1. A fA-level weak signal detection device for terahertz spectroscopy and imaging system, characterized in that: It includes a TIA pre-current amplifier, a pre-gain amplifier, a BPF band-pass filter, a band-pass gain amplifier, a MODEM modem, an LPF low-pass filter, a low-pass gain amplifier, an AD analog-to-digital converter, a POWER power supply module, a main control MCU and a PC host computer; the TIA pre-current amplifier input end is connected to the detection antenna of the current-type terahertz spectrum and imaging system, and is used to collect and amplify the bias current signal generated by the detection antenna; the pre-gain amplifier input end is connected to the TIA pre-current amplifier output end, or is connected to the detection antenna of the voltage-type terahertz spectrum and imaging system, and is used to further amplify the input signal; the BPF band-pass filter input end is connected to the pre-gain amplifier output end, and is used to filter out high-frequency and low-frequency noise; the band-pass gain amplifier input end is connected to the BPF band-pass filter output end, and is used to further amplify the input signal; the MODEM modem input end is respectively connected to the band-pass gain amplifier output end The device is connected to the main control MCU and is used to multiply the reference signal input by the main control MCU with the signal input by the bandpass gain amplifier, and output a DC signal. The reference signal has the same frequency as the input signal of the detection antenna. The input end of the LPF low-pass filter is connected to the output end of the MODEM modem, and is used to filter out the AC noise signal and output a DC useful signal. The input end of the low-pass gain amplifier is connected to the output end of the LPF low-pass filter, and is used to further amplify the signal. The input end of the AD analog-to-digital converter is connected to the output end of the low-pass gain amplifier, and is used to convert the analog signal into a digital signal, and transmit the digital signal to the main control MCU. The main control MCU is respectively connected to the MODEM modem, the AD analog-to-digital converter and the PC host computer, and is used to output a reference signal with the same frequency as the bias current signal, receive the digital signal sent by the AD analog-to-digital converter and upload it to the PC host computer, and the PC host computer performs data processing and presents a terahertz spectrum in the form of a time domain waveform and a frequency domain waveform.

2. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1, characterized in that: The fA-level weak signal detection device for terahertz spectroscopy and imaging system also includes a shell, in which a TIA pre-current amplifier, a pre-gain amplifier, a BPF band-pass filter, a band-pass gain amplifier, a MODEM modem, an LPF low-pass filter, a low-pass gain amplifier, an AD analog-to-digital converter, a POWER power supply module and a main control MCU are all built-in.

3. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1, characterized in that: The POWER power supply module is respectively connected with the TIA pre-current amplifier, the pre-gain amplifier, the BPF band-pass filter, the band-pass gain amplifier, the MODEM modem, the LPF low-pass filter, the low-pass gain amplifier, the AD analog-to-digital converter and the main control MCU.

4. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1, characterized in that: The TIA pre-current amplifier uses fA-class op amps, with input bias current as low as 20fA, offset voltage as low as 50uV, low offset voltage drift, and adjustable gain of 1*10. ^4 Up to 1*10 ^9 , voltage noise density can be as low as 14nV / √ Hz(10KHz), up to 2MHz bandwidth.

5. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1, characterized in that: The amplification factor of the pre-gain amplifier is 1, 2, 5 or 10 times, the amplification factor of the band-pass gain amplifier is 1, 2, 5 or 10 times, the amplification factor of the low-pass gain amplifier is 2, 5, 10 or 20 times, and the cut-off frequency of the LPF low-pass filter can be selected to be 3Hz, 10Hz, 30Hz or 100Hz.

6. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1 or 5, characterized in that: The PC host computer includes a phase adjustment module, a three-level gain adjustment module, a low-pass cutoff frequency adjustment module and a display module. The phase adjustment module is used to adjust the phase of the reference signal, and the phase adjustment range is 0-360°; the three-level gain adjustment module is used to adjust the amplification factors of the pre-gain amplifier, the band-pass gain amplifier and the low-pass gain amplifier; the low-pass cutoff frequency adjustment module is used to adjust the cutoff frequency of the LPF low-pass filter. The pre-gain amplifier, the band-pass gain amplifier, the low-pass gain amplifier and the LPF low-pass filter are all connected to the main control MCU. The display module is used to input and display the adjusted phase degree, gain speed and frequency, and display the terahertz spectrum in real time.

7. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1 or 5, characterized in that: A dip switch and a rotary encoder are arranged on the shell panel. The dip switch is respectively connected to the pre-gain amplifier, the band-pass gain amplifier, the low-pass gain amplifier and the LPF low-pass filter, and is used for adjusting the gain of the pre-gain amplifier, the band-pass gain amplifier and the low-pass gain amplifier and the cut-off frequency of the LPF low-pass filter. The rotary encoder is internally connected to the main control MCU and is used for inputting the adjusted phase value into the main control MCU, and the main control MCU outputs a reference signal for phase adjustment.

8. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 1, characterized in that: The housing panel is provided with a voltage collection input port VIN and a current collection input port CIN, wherein the voltage collection input port VIN is internally connected to a TIA pre-current amplifier and externally connected to a detection antenna for outputting current signals, and the current collection input port CIN is internally connected to a pre-gain amplifier and externally connected to a detection antenna for outputting voltage signals.

9. The fA-level weak signal detection device for terahertz spectroscopy and imaging system according to claim 5, characterized in that: An analog voltage output port OUT is provided on the shell panel, and the analog voltage output port OUT is internally connected to the output end of the low-pass gain amplifier. A modulation signal output port CLK is provided on the shell panel, and the modulation signal output port CLK is internally connected to the reference signal output end of the main control MCU. A USB interface is provided on the shell panel, and the main control MCU is connected to the PC host computer through the USB interface. The shell panel is provided with an OFF / ON button for starting and shutting down the device, a power input port (24V) and fault, power and status indicator lights.

10. A fA-level weak signal detection method for terahertz spectroscopy and imaging system, characterized in that: The implementation process is as follows: 1) The current signal received by the detection antenna is first amplified by the TIA pre-current amplifier; 2) After passing through the TIA pre-current amplifier, the signal enters the pre-gain amplifier for further amplification; 3) After the pre-gain amplifier signal is output, it enters the BPF bandpass filter to filter out high-frequency and low-frequency noise; 4) After the BPF bandpass filter output, the signal enters the bandpass gain amplifier for further amplification; 5) The phase of the reference signal output by the main control MCU is adjusted through a PC host computer or a rotary encoder. The phase adjustment range is 0-360°. The reference signal is at the same frequency as the input signal output by the detection antenna. 6) After passing through the bandpass gain amplifier, the signal enters the MODEM modem. At this time, the main control MCU provides a reference signal to perform multiplication operation with the input signal; 7) The output DC signal enters the LPF low-pass filter to filter out the AC noise signal and output a DC useful signal; 8) The signal after passing through the LPF low-pass filter enters the low-pass gain amplifier for further amplification; 9) After low-pass gain, the signal passes through an AD analog-to-digital converter to convert the analog signal into a digital signal. After the main control MCU collects the digital signal, the data is uploaded to the PC host computer. After the PC host computer processes the data, it is presented in the form of time domain waveform and frequency domain waveform. Repeat steps 5)-9) until the extreme value difference of the spectrum output by the PC host computer is the largest, and the adjustment is completed.

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