CN110220864B - Method for improving signal-to-noise ratio of time-resolved terahertz spectrum by using double phase-locking technology - Google Patents
Method for improving signal-to-noise ratio of time-resolved terahertz spectrum by using double phase-locking technology Download PDFInfo
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Abstract
The invention relates to a method for improving the signal-to-noise ratio of a time-resolved terahertz spectrum by using a double phase-locking technology, which comprises the steps of firstly, building a time-resolved terahertz spectrum system; secondly, scanning a delay line to complete signal input of a detection part; then inputting the signals of the detection part into the two phase-locked amplifiers, and simultaneously completing the signal synchronization between the two choppers and the two phase-locked amplifiers; then, synchronizing signals of the chopper in the terahertz generation part to the chopper in the pump part, inputting the signals of which the background noise of the pump light path is filtered into the phase-locked amplifier in the generation part from the phase-locked amplifier, subtracting the signals of which the background noise is filtered, finally obtaining a signal difference value delta E/E through demodulation of the phase-locked amplifier in the generation part, and scanning a delay line to obtain the complete carrier dynamics process of the sample. The invention can greatly improve the signal-to-noise ratio of the system, can be used in a time detection terahertz system and is also suitable for occasions with small signals and strong background noise.
Description
Technical Field
The invention belongs to the technical field of improving the signal-to-noise ratio of a time-resolved terahertz spectrum in an ultrafast spectrum technology, and particularly relates to a method for improving the signal-to-noise ratio of the time-resolved terahertz spectrum by using a double phase-locking technology.
Background
The terahertz spectrum technology is an effective means for spectrum research in the terahertz waveband, and can be used for researching the characteristics of substances in various fields such as biology, environment, medicine, security inspection, communication and the like. The time-resolved terahertz spectrum is an effective non-contact detection method developed in recent years by combining a terahertz time-domain spectrum and a pumping-detection technology, information reflected by photoinduced change of a sample signal can be directly observed, and a research is carried out on a material in-band charge transfer process. Compared with the terahertz time-domain spectrum which can only measure the static characteristics of the sample, the time-resolved terahertz spectrum can measure the dynamic change information of the substance.
In the time-resolved terahertz system, one beam of ultrafast laser pulse is divided into three beams in the light splitting process, the three beams are used for generating exciting light, pumping light and detection light of terahertz radiation, and compared with terahertz time-domain spectroscopy, one more beam of pumping light path is divided out to serve as a pumping source to excite a sample. The optical path contains two delay lines, which can not only observe the ultrafast phenomenon of carrier movement in the semiconductor sample, but also detect the change of the terahertz spectrum of the sample under the optical pumping. Therefore, the commercialization of the time-resolved terahertz spectrum also becomes a main development direction of the current terahertz technology, which also puts higher requirements on the speed and precision of generation, detection, transmission and processing of terahertz signals.
Due to the application of femtosecond laser pulses, terahertz signals are generated and detected, but the energy ratio of the detected signals to noise is about 1:106, so that the terahertz signals are easily buried in background noise, and certain measures need to be taken to suppress the noise and improve the signal quality. Aiming at the higher requirement of signal-to-noise ratio of the signal acquisition, a method for removing background noise and extracting effective signals, namely a phase-locked amplification technology, is mainly provided at present.
The phase-locked amplification technique is a common technique for extracting a required signal from background noise by an instrument, and the instrument for realizing the technique is a phase-locked amplifier. The phase-locked amplifier can be used for extracting weak signal detection in strong noise, has the advantages of automatically acquiring the size and direction of a detected signal, having strong anti-interference capability and greatly improving the measurement precision of the detected signal, and is widely applied to signal detection. A common phase-locked amplifying circuit mainly comprises a signal channel, a reference channel, a phase detector and a low-pass filter. The lock-in amplifier used in the time-resolved terahertz system is an SR 830. The terahertz signal processing instrument is also a core instrument for processing terahertz signals, and can filter most of noise, so that parameter selection of the terahertz signal processing instrument also plays a certain role. The phase-locked amplifier mainly plays the roles of low-pass filtering and phase-sensitive detection amplification in the system. The low-pass filtering process requires setting a corresponding low-pass time constant, which is closely related to the quality of signal acquisition. The reference signal input of the phase sensitive detection amplified part is provided by a chopper. However, the signal acquisition method adopted by the current time resolution terahertz system is generally only applied to one phase-locked amplifier, and the signal-to-noise ratio is not high enough, so that effective information noise is annihilated and material characteristics and ultra-fast dynamics information cannot be accurately acquired.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a method for improving the signal-to-noise ratio of a time-resolved terahertz spectrum by using a double phase-locking technology; the adopted double phase-locking technology can effectively remove background noise caused by laser energy jitter in the time-resolved terahertz spectrum, improve the signal-to-noise ratio and acquire effective physical information. The method only depends on optimization and information modulation in the signal acquisition process, and has the advantages of easy implementation and capability of greatly optimizing the signal-to-noise ratio.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for improving the signal-to-noise ratio of a time-resolved terahertz spectrum by using a double-phase-locking technology comprises the steps of firstly, constructing a complete time-resolved terahertz spectrum system comprising a terahertz generation part, a terahertz detection part, a pumping part and a phase-locking amplification part; secondly, scanning a delay line to complete signal input of a detection part; then inputting the signals of the detection part into the two phase-locked amplifiers, and simultaneously completing the signal synchronization between the two choppers and the two phase-locked amplifiers; then, signals of the chopper in the terahertz generating part are synchronized with signals of the chopper in the pumping part, the signals with the filtered pumping light path background noise are input into the phase-locked amplifier in the generating part from the phase-locked amplifier, the difference is made between the signals with the filtered pumping light path background noise and the signals with the filtered pumping light path background noise, a signal difference value delta E/E is finally obtained through demodulation of the generating part phase-locked amplifier, and the complete carrier dynamics process of the sample can be obtained through scanning a delay line. The method comprises the following specific steps:
101. a time resolution terahertz spectrum system is built: the femtosecond laser is divided into three beams of light by two beam splitters, and the three beams of light are respectively used for generating and detecting terahertz pulses and pumping samples;
102. collecting signals of the sample after pumping: information of the terahertz pulses changing along with time after the sample is pumped is obtained by scanning the delay line 2 in the pumping part and enters the detection part to be collected.
103. The signal is accessed to the phase-locked amplifying part and completes signal modulation: the detector receives two beams of linearly polarized signals, and the two beams of linearly polarized signals are respectively input into the phase-locked amplifiers #1 and # 2; the reference signals of the chopper in the simultaneous generation part and the pumping part are also input into the phase-locked amplifiers #1 and #2, so that the selection of the same-frequency and same-phase signals is realized.
104. And outputting the signal difference value delta E/E: and a reference signal of the chopper 1 in the generation part is input into the chopper 2 in the pumping part, so that the synchronization of the two choppers is realized. The signal input into the phase-locked amplifier #2 is input into the phase-locked amplifier #1 after the background noise of the pumping part is filtered by the frequency selected by the chopper 2, the difference is made between the signal generated after the background noise of the pumping part is filtered by the chopper 1 and the signal generated after the background noise of the phase-locked amplifier #1 is filtered, and then the signal delta E/E is finally obtained through demodulation.
105. The signal delta E/E is amplified and then input into a computer to be collected: and obtaining a complete carrier kinetic process of the sample under the excitation of light.
Compared with the prior art, the invention has the following prominent substantive characteristics and remarkable advantages:
the method adopts the double phase-locking technology to improve the signal-to-noise ratio, can effectively extract the effective signals in the time-resolved terahertz spectrum, avoids the influence of energy jitter of a laser on the measured signals, can greatly improve the signal-to-noise ratio of the system, and has important significance for the accuracy of extracting material properties and physical information essence when the spectroscopic system is applied to the field of materials. Meanwhile, the method is also suitable for the signal acquisition process in terahertz signal acquisition or other small signal strong background occasions.
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FIG. 1 is a flow chart of the method of the present invention.
Fig. 2 is a schematic diagram of an experimental optical path and signal acquisition of a time-resolved terahertz spectroscopy system.
Detailed Description
The following detailed description of specific embodiments of the invention refers to the accompanying drawings.
As shown in fig. 1, a method for improving the signal-to-noise ratio of a time-resolved terahertz spectrum by using a double phase-locking technique includes the following specific steps: firstly, a complete time resolution terahertz spectrum system comprising a terahertz generation part, a terahertz detection part, a pumping part and a phase-locked amplification part is required to be built; secondly, scanning a delay line to complete signal input of a detection part; then inputting the signals of the detection part into the two phase-locked amplifiers, and simultaneously completing the signal synchronization between the two choppers and the two phase-locked amplifiers; then, synchronizing signals of the chopper in the terahertz generation part to the chopper in the pump part, inputting the signals of which the background noise of the pump light path is filtered into the phase-locked amplifier in the generation part from the phase-locked amplifier, subtracting the signals of which the background noise is filtered, finally obtaining a signal difference value delta E/E through demodulation of the phase-locked amplifier in the generation part, and scanning a delay line to obtain the complete carrier dynamics process of the sample.
Referring to fig. 2, the embodiment is built on a time-resolved terahertz spectroscopy system, and is composed of four parts, including a terahertz generation part, a pumping part, a terahertz detection part, and a phase-locked amplification part; in the system, a femtosecond laser (energy 3mJ, laser pulse width 35fs) is processed according to the proportion of 2: 1 is divided into two paths, one path of laser with the energy of 2mJ passes through an optical delay line 2 to provide a time dynamic window more than 1ns, and the laser is contracted and irradiated on a sample after passing through the optical delay line to form a sample pumping part; and the other beam of 1mJ laser is used as a medium through air plasma induced by the laser, and the time-resolved detection of the broadband terahertz wave is realized by measuring a second harmonic signal generated by the induction of the terahertz wave field.
Firstly, 800nm fundamental frequency light passes through a beta-BBO frequency doubling crystal, wherein the residual fundamental frequency light and second harmonic generated by frequency doubling are focused in the air through a convex lens to generate air plasma, and broadband terahertz waves with high intensity are generated through a three-order nonlinear four-wave frequency mixing optical process to form a terahertz generation part; and then, detecting the time domain waveform of the terahertz by using an electro-optic sampling method, and resolving the terahertz pulse by adjusting the angle of the electro-optic crystal to form a terahertz detection part.
The detection part principle in the experiment is that the electro-optical crystal generates a second-order polarization effect under the action of a terahertz radiation electric field, and when a detection light pulse passes through the electro-optical crystal, the detection light pulse carries information of changed polarization state, is incident on the Wollaston prism through the 1/4 wave plate, is divided into two beams of linear polarization light which are perpendicular to each other, and is received by the two detectors (photodiodes). According to the intensity signal difference value of the two beams of linearly polarized light detected by the two photodiodes, the electric field information of the terahertz wave can be obtained.
The electro-optic crystal commonly used in the experiment is ZnTe, and the refractive index of ZnTe in near infrared and far infrared wave bands is equivalent, so that the phase matching of terahertz and detection light is easy to realize. And then two choppers are combined with a phase-locked amplifier, so that the influence of laser energy jitter on an experimental result is eliminated, and the signal-to-noise ratio is further improved.
The terahertz pulse generated by the generation part carries sample information after passing through a sample, an output signal is obtained by the detection part, an original signal contains a large amount of millivolt background noise besides a microvolt useful signal, and therefore certain amplification is needed, but if the original signal is directly amplified, environmental noise can be amplified along with the ambient noise, so that band-pass filtering is needed to be carried out firstly, and a large amount of signals in a non-modulation frequency domain are removed and then amplified.
In the time-resolved terahertz spectrum, in order to measure the terahertz spectrum before and after the excitation of a sample, firstly, scanning the time delay between detection light and a terahertz signal through a time delay line 1, and fixing a measurement signal at the maximum position of the absolute value of the amplitude of the terahertz signal; then, the delay line 2 in the pumping part is used for scanning the time delay between the pumping light path and the terahertz generation light path, and scanning the terahertz signal peak value change before and after the pumping light excitation, so that the obtained time-resolved terahertz spectrum also contains background noise caused by laser energy oscillation in the pumping light path, which is also required to be avoided. Therefore, the signal ratio of system signal acquisition is improved by adopting a double phase-locking technology, and the method comprises the following specific steps:
the basic principle of the phase-locked amplification technology is to utilize the cross-correlation detection principle to detect a signal V to be detectedSAnd a reference signal VRFirst multiplying to obtain a product signal V1And then output by low pass filtering.
Wherein the signal V to be measuredSContains various frequency components and can be expressed as VS＝∑Ei sin(2πfit+θi) In which Ei、fi、θiRespectively representing the amplitude, frequency and phase of each signal component to be detected; reference signal VRFor selecting frequency signals, it can be expressed as VR＝∑Er sin(2πfrt+θr) In which E isr、fr、θrRepresenting the amplitude of each reference signal component separatelyValue, frequency, phase magnitude;
product signal V1It can be expressed as formula (1):
if VSContaining a frequency component fx＝frV output by low-pass filtering0Can be expressed by equation (2):
that is, the component of the measured signal with the same frequency as the reference signal is retained, and other frequency components are suppressed.
In a general terahertz time-domain spectrum, a chopper and a lock-in amplifier are combined to realize signal optimization. The chopper acting as a frequency f to the lock-in amplifierrAlso as a reference signal of frequency f for introducing terahertzxThe modulation signal in the terahertz signal is reserved, background noise without modulation noise is filtered, and effective information ratio in the output signal is improved.
The double phase-locking technique works based on the modulation of the selected carrier frequency for the corresponding optical excitation, compared to the common single-channel phase-locking technique. The double chopper is combined with two phase-locked amplifiers to respectively control a pump light path and an exciting light path, so that the influence of energy jitter of a laser on an acquired signal is eliminated, and the signal-to-noise ratio is further improved; the first phase locking is the same as the previous phase locking amplification process, and the frequency of the chopper generating the light path to the phase locking amplifier 1 is fr1For the terahertz pulse signalr1A reference signal of frequency; in the second phase-locking, the chopper 2 in the pump beam path gives the phase-locked amplifier 2 a frequency fr2Is also subject to the output frequency f of the chopper 1r1Modulation of (3). The generation of terahertz detection pulses should be adjustedTo a higher carrier frequency. And in the second phase locking, the frequency of light excitation of the material is adjusted to be about 10 times lower by using a chopper.
Previous researches show that the conductivity of a sample in a terahertz frequency domain is generally determined by the combined action of carrier concentration and carrier mobility, so that in order to more intuitively research the change condition of the carrier concentration in the sample, the evolution process of an ultrafast dynamic process in the sample is further researched through the change of the conductivity of the sample. After a sample is excited by pump light, rapid and instantaneous rise of a terahertz signal can be observed, and the change | Delta E/E | of a peak signal is proportional to the real part sigma of the terahertz conductivity sigmarealAs in equation (3):
wherein n issubIs the refractive index of the sample substrate, Z0The fixed value is 377 omega for free space impedance.
The peak signal Δ E/E in the above formula (3) is the signal to be acquired. Referring to fig. 1 and 2, signals collected by two photodiodes in the detection part are input to a lock-in amplifier #1 and a lock-in amplifier #2 via signal input ports 1 and 2, respectively, and then are input to two choppers at a frequency fr1、fr2As the frequency of the reference signal, the reference signal ports 1 and 2 of the choppers 1 and 2 are respectively input into the phase-locked amplifiers #1 and #2 through the reference signal input ports 1 and 2, so that the signal component with the same frequency as the reference signal given by the chopper in the terahertz signal is reserved, and the background noise signal is suppressed; frequency f of chopper 2 following pump beam pathr2The frequency of a synchronous modulation signal of a chopper 1 in the terahertz generation optical path is also used, and a synchronous signal is input into a synchronous input signal port 2 from a synchronous output signal port 1 to ensure the synchronization of signal acquisition; then, the signal of the phase-locked amplifier #2 is output to the phase-locked amplifier #1 from the signal output port 2 through the auxiliary input port 1, and the final collection of the peak signal delta E/E is realized. Finally, according to a formula (3), the terahertz frequency domain of the sample can be obtainedThe change in internal conductivity, in turn, reduces the change in internal charge transfer in the sample.
Claims (1)
1. A method for improving the signal-to-noise ratio of a time-resolved terahertz spectrum by using a double-phase-locking technology is characterized in that firstly, a complete time-resolved terahertz spectrum system comprising a terahertz generation part, a terahertz detection part, a pumping part and a phase-locking amplification part is required to be built; the femtosecond laser is processed according to the proportion of 2: 1 is divided into two paths, one path of laser with larger energy passes through the optical delay line 2, and then is irradiated on a sample after beam shrinking to form a pumping part of the sample; the other laser beam is used as a medium through air plasma induced by the laser, and the time-resolved detection of the broadband terahertz wave is realized by measuring a second harmonic signal generated by the induction of the terahertz wave field; allowing the fundamental frequency light to pass through a beta-BBO frequency doubling crystal, wherein the residual fundamental frequency light and second harmonic generated by frequency doubling are focused in the air through a convex lens, generating air plasma, and generating broadband terahertz waves with high strength through a three-order nonlinear four-wave frequency mixing optical process to form a terahertz generation part; detecting a terahertz time-domain waveform by using an electro-optic sampling method, and resolving terahertz pulses by adjusting the angle of an electro-optic crystal to form a terahertz detection part; the double chopper combines two phase-locked amplifiers to respectively control a pump light path and an exciting light path, eliminates the influence of laser energy jitter on the acquired signals and forms a phase-locked amplifying part; the method comprises the following specific steps:
101. a time resolution terahertz spectrum system is built: the femtosecond laser is divided into three beams of light by two beam splitters, and the three beams of light are respectively used for generating terahertz pulses, detecting the terahertz pulses and pumping samples;
102. collecting signals of the sample after pumping: the terahertz pulse generated by the generation part carries sample information after passing through a sample, and an output signal is obtained by the detection part; information of the terahertz pulses changing along with time after the sample is pumped is obtained by scanning a delay line 2 in the pumping part and enters the detection part to be collected;
103. the signal collected by the detection part is accessed to the phase-locked amplification part and completes signal modulation: the electro-optic crystal generates a second-order polarization effect under the action of a terahertz radiation electric field, when a detection light pulse passes through the electro-optic crystal, the detection light pulse carrying information of a changed polarization state is incident on the Wollaston prism through an 1/4 wave plate and is divided into two beams of linear polarization light which are perpendicular to each other, and a detector of a detection part receives signals of the two beams of linear polarization light and respectively inputs the signals into phase-locked amplifiers #1 and # 2; meanwhile, reference signals of choppers in the generating part and the pumping part are also input into the phase-locked amplifiers #1 and #2, so that the selection of same-frequency and same-phase signals is realized;
104. and outputting the signal difference value delta E/E: inputting a reference signal of a chopper 1 in the generation part into a chopper 2 in the pumping part to realize the synchronization of the two choppers; the signal input into the phase-locked amplifier #2 is input into the phase-locked amplifier #1 after the background noise of the pumping part is filtered by the frequency selected by the chopper 2, the difference is made between the signal generated after the background noise of the pumping part is filtered by the chopper 1 and the signal generated after the background noise of the phase-locked amplifier #1 is filtered, and the peak value signal delta E/E is finally obtained through demodulation;
105. the peak value signal delta E/E is amplified and then input into a computer to be collected: obtaining a complete carrier kinetic process of the sample under the excitation of light;
after the sample is excited by pump light, the rapid and instantaneous rise of the terahertz signal can be observed according to a formula:
wherein σrealIs the real part of the terahertz conductivity sigma, nsubIs the refractive index of the sample substrate, Z0The impedance of free space is taken as a constant value 377 omega, and the peak value signal delta E/E in the formula is the signal to be collected.
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| CN110220864A (en) | 2019-09-10 |
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