A hyper-spectral detection method based on heterodyne technique and an on-line hyper-spectral detector adopting the method

Abstract

The present application relates to a kind of based on heterodyne technique's hyperspectral detection method and the online hyperspectral detector using this method, belong to photoelectric detection technical field, solve the technical problem that the detection spectral range is limited.The method includes the following steps: after the optical collimation system collimation of the light signal to be measured, enter optical fiber and couple;Local light laser of the frequency band to be measured emits local light signal, and is divided into probe signal and reference signal by optical beam splitter.For the frequency band below wavelength 760nm, when the central wavelength of laser is not tunable, the present application utilizes the central frequency of band-pass filter to change, realizes the ultra-high resolution spectrum scanning of laser frequency band below 760nm.The present application changes from limited to possible for the heterodyne detection method of laser wavelength non-tunable range, and the implementation method is simple, the pressure of laser drive is changed into the implementation pressure of electronic filter, and digital control method is more flexible.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of photoelectric detection, and particularly relates to a heterodyne technology-based hyperspectral detection method and an online hyperspectral detector adopting the method. BACKGROUND

[0002] Laser heterodyne spectroscopy is a coherent detection, and direct detection is the counterpart thereof. Both are main detection modes of photoelectric detection. Compared with the direct detection mode, in the laser heterodyne detection, the local light and the detection light are simultaneously irradiated onto the photosensitive surface of the detector, and the two lights are matched with each other to form a beat frequency electric signal.

[0003] Since the spectral signal obtained by using the laser heterodyne detection technology has high spectral resolution, since the 1970s, the laser heterodyne detection technology has been gradually applied to the fields of atmospheric detection, astronomy and water observation, and has gradually played an important role.

[0004] When applied to the field of hyperspectral detection, the center wavelength of the local light source is taken as the starting point of the required detection characteristic spectral band window, and the wavelength of the characteristic spectral band is swept by current driving, and the hyperspectral data acquisition with high resolution is realized by cooperating the detector with the bandpass filter. For example, the typical applications at present include the ground-based solar methane atmospheric column distribution detection by using a DFB laser in the near-infrared band, and the high-resolution emission spectrum detection of the atmospheric carbon dioxide and ozone of Venus and Mars by using a QCL laser.

[0005] According to the technical principle, the laser heterodyne high-resolution detection technology has a spectral resolution of picometer level or even sub-picometer level, which can meet the stringent requirements of most scientific detection on spectral resolution. However, at present, the spectral detection range of the laser heterodyne detection technology is mainly limited by the wavelength tunable range of the used laser. For example, when a near-infrared DFB laser is used, the current driving tunable range is generally less than 1 nm, and at this time, the spectral detection range of the laser heterodyne detection technology is less than 1 nm.

[0006] Figure 2The traditional laser heterodyne push-broom process is shown, the bandwidth of the detector determines the wavelength range of the received signal, and the center wavelength of the local light laser determines the wavelength position of the signal to be measured. When the signal to be measured is a wide spectrum light, the signal to be measured needs to be filtered. A band-pass filter is usually used to filter out a narrow-band signal, combined with the laser push-broom function, the center frequency of the laser is changed, the band-pass filter changes the center frequency of the band-pass filter according to the change of the center frequency of the laser, and a certain frequency difference is maintained between the center frequency of the laser and the center frequency of the band-pass filter. The push-broom process is a super high resolution spectrum scanning process, the spectrum resolution is equal to the bandwidth of the band-pass filter, and the scanning range is equal to the push-broom range of the laser. For example, the starting shortest wavelength of the DFB laser in the tunable laser is 760nm. When performing super high resolution spectrum scanning, the center frequency of the band-pass filter needs to be adjusted to maintain a certain frequency difference with the center frequency of the DFB laser. Therefore, the super high resolution spectrum scanning in the prior art relies on the laser push-broom function, and cannot meet the detection requirements of wavelengths less than 760nm. For example, the biological detection spectrum range is generally between 600-700nm, and the tunable range of the DFB laser in the tunable laser cannot cover the above frequency band, and the super high resolution spectrum scanning cannot be performed in the above frequency band. SUMMARY

[0007] The present application solves the technical problem of the limited spectrum range of the laser heterodyne detection technology at the present stage, and provides a super spectrum detection method based on the heterodyne technology and an online super spectrum detector using the method.

[0008] In order to solve the above technical problems, the technical scheme of the present application is as follows:

[0009] A super spectrum detection method based on the heterodyne technology can realize multi-band spectrum scanning without relying on a tunable laser as a local light generator. The applicable detection system includes a band-pass filter with a variable center frequency and a local light generator.

[0010] The band-pass filter is used to change the center frequency to perform spectrum scanning of the measured frequency band, and the band-pass width can be changed to modify the wavelength width of the spectrum window.

[0011] The local light generator is used to emit a local light signal of the measured frequency band. The local light generator can be a tunable laser or a non-tunable laser.

[0012] The super spectrum detection method includes the following steps:

[0013] Step 1: coupling the measured light signal with the local light signal;

[0014] Step 2: the coupled optical signal is processed into an electric signal heterodyne signal;

[0015] Step 3: the optical spectrum scanning of the to-be-detected frequency band is performed; the center frequency and the bandwidth of the band-pass filter are changed, thereby realizing frequency selection and bandwidth regulation of the electric signal heterodyne signal, and the optical spectrum amplitude signal is output after calculation.

[0016] In the above technical solution, the detection system suitable for the hyperspectral detection method specifically comprises, in sequence: a telescope system, an optical collimation system, a first optical fiber, a high-speed photoelectric detector, a radio frequency signal processing module, a radio frequency detector, a signal acquisition / generation module, and a central CPU module;

[0017] The signal acquisition / generation module is further connected, in sequence, with a local oscillator light generator, a second optical fiber, a beam splitter, an attenuator, and a calibration photoelectric detector; the calibration photoelectric detector is further connected with the signal acquisition / generation module;

[0018] The radio frequency signal processing module is provided with a band-pass filter with a variable center frequency;

[0019] The hyperspectral detection method comprises the following steps:

[0020] Step I: the to-be-detected optical signal is received by the telescope system, collimated by the optical collimation system, and then enters the first optical fiber;

[0021] Step II: the central CPU module generates a digital signal and sends it to the signal acquisition / generation module, to drive the local oscillator light generator to emit a local oscillator light signal of the to-be-detected frequency band; the local oscillator light signal enters the beam splitter through the second optical fiber, and is divided into a detection signal and a reference signal by the beam splitter;

[0022] Step III: the detection signal of the local oscillator light signal is coupled with the to-be-detected optical signal in the first optical fiber and is received by the high-speed photoelectric detector; the reference signal of the local oscillator light signal is transmitted to the calibration photoelectric detector after passing through the attenuator, and the 3rd harmonic is extracted after processing by the signal acquisition / generation module and the central CPU module, to feed back and calibrate the center wavelength of the local oscillator light;

[0023] Step IV: the coupled optical signal generates an electric signal heterodyne signal after passing through the high-speed photoelectric detector; the light with a center frequency different from that of the local oscillator light within the fixed bandwidth of the high-speed photoelectric detector passes through the high-speed photoelectric detector;

[0024] Step V: the electric signal heterodyne signal is screened through the band-pass filter, and the signal with the same bandwidth as the band-pass width of the band-pass filter is the channel bandwidth of the optical spectrum scanning channel; the center frequency of the band-pass filter is changed to realize the optical spectrum scanning of the to-be-detected frequency band; the signal amplified by the radio frequency after frequency selection is obtained after processing by the radio frequency signal processing module;

[0025] Step VI: the signal amplified by the radio frequency is received by the radio frequency detector, collected by the signal collection / generation module, fed back to the central CPU module, and the spectral amplitude signal is output after calculation by the central CPU module.

[0026] In the above technical solution, steps I and II are performed simultaneously.

[0027] In step V, the heterodyne signal is processed by the radio frequency signal processing module, specifically:

[0028] First, the direct current is removed through the direct current isolation circuit;

[0029] Then, the signal is screened through the band-pass filter;

[0030] Finally, the selected signal is amplified by the radio frequency.

[0031] In the above technical solution, the wavelength of the local oscillator light signal can be less than 760nm, i.e. a wavelength-unadjustable laser can be selected.

[0032] In the above technical solution, the spectral resolution is the passband bandwidth of the band-pass filter, and both the bandwidth and the center frequency can be adjusted.

[0033] In the above technical solution, the center frequency variable band-pass filter uses the central CPU module to control the peripheral relay circuit to realize the parallel connection of the capacitor and the resistor in the band-pass filter, thereby changing the capacitive reactance and impedance value of the band-pass filter to realize the change of the center frequency.

[0034] An online hyperspectral detector, which adopts the above-mentioned hyperspectral detection method when detecting.

[0035] The present application has the following advantages:

[0036] The hyperspectral detection method based on heterodyne technology of the present application changes the heterodyne detection method for laser wavelength-unadjustable range from limited to possible, and the implementation method is simple, which changes the adjustable hard requirement of changing the center wavelength of the laser into the function realization of the electronic filter, and the digital control method is more flexible.

[0037] The online hyperspectral detector of the present application realizes the ultra-high resolution spectral scanning of the laser frequency band below 760nm by changing the center frequency of the band-pass filter when the center wavelength of the laser is unadjustable (such as the frequency band below 760nm).

[0038] The present stage visible light tuning range is generally about 10pm, the wide tuning range visible light semiconductor laser generally has a built-in PZT (piezoelectric ceramic), which is equivalent to a cavity tuning technology, and the complexity and cost are relatively high, and the realization of the hyperspectral detection method of the present application can break through the dependence of the heterodyne technology on the tuning range of the laser.

[0039] The online hyperspectral detector of the present application selects a customized single-mode detector for the photodetector, can realize effective wavelength coverage of 400nm-1100nm for ultrahigh-resolution spectral scanning, and can realize high-resolution spectral scanning covering wavelengths including 300nm-1700nm. BRIEF DESCRIPTION OF DRAWINGS

[0040] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0041] Figure 1 The structure diagram of the detection system suitable for the hyperspectral detection method of the present application realized based on the center wavelength variable bandpass filter.

[0042] Figure 2 The schematic diagram of the laser heterodyne detection method for realizing hyperspectral push-broom in the prior art based on laser driving.

[0043] Figure 3 The flowchart of the hyperspectral detection method of the present application. DETAILED DESCRIPTION

[0044] The hyperspectral detection method of the present application is based on the traditional laser heterodyne detection technology, and in some specific wavebands (such as the visible light waveband) where the laser tuning range is insufficient, a center frequency variable bandpass filter is used to realize the expansion of the detection range, i.e. the spectral scanning function; at the same time, the bandpass filter has a tunable bandpass width, i.e. the spectral resolution can be adjusted simultaneously, and in the actual application process, the spectral resolution and the effective signal signal-to-noise ratio can be considered. On the basis of the existing hyperspectral resolution advantage, the spectral detection range of the laser heterodyne detection technology is effectively expanded, and the application in the fields of atmospheric remote sensing, water marks, astronomical detection, etc. has important practical value.

[0045] The detection system suitable for the hyperspectral detection method based on the heterodyne technology of the present application comprises a center frequency variable bandpass filter and a local oscillator light generator; the center frequency variable bandpass filter is used to perform spectral scanning of the to-be-detected frequency band by changing the center frequency; and the local oscillator light generator is used to emit a local oscillator light signal of the to-be-detected frequency band.

[0046] The hyperspectral detection method based on the heterodyne technology of the present application comprises the following steps:

[0047] Step 1: coupling the to-be-detected light signal with the local oscillator light signal;

[0048] Step 2: processing the coupled optical signal into a heterodyne signal of an electrical signal;

[0049] Step 3: performing spectral scanning of the to-be-detected frequency band; changing the center frequency and bandwidth of the band-pass filter, thereby realizing frequency selection and bandwidth regulation of the heterodyne signal of the electrical signal, and outputting a spectral amplitude signal after calculation.

[0050] The application will be described in detail below with reference to the drawings.

[0051] As shown in Figure 1 , the super-spectral detection method based on the heterodyne technology of the application is applicable to a detection system, which specifically comprises, in sequence: a telescopic system, an optical collimation system, a first optical fiber, a high-speed photoelectric detector, a radio frequency signal processing module, a radio frequency detector, a signal acquisition / generation module, and a central CPU module; the signal acquisition / generation module is further connected, in sequence, with: a local oscillator light generator, a second optical fiber, a beam splitter, an attenuator, and a calibration photoelectric detector; the calibration photoelectric detector is further connected with the signal acquisition / generation module; and the radio frequency signal processing module is provided with a band-pass filter with a variable center frequency. The local oscillator light generator comprises: a local oscillator light generation module and a local oscillator laser, and the local oscillator light generation module is used to drive the local oscillator laser to emit a local oscillator light signal of a to-be-detected frequency band according to a driving signal received thereby.

[0052] As shown in Figure 1 and 3 , the super-spectral detection method based on the heterodyne technology of the application comprises the following steps:

[0053] Step I: after the telescopic system receives a to-be-detected optical signal, the optical signal is collimated by the optical collimation system and then enters the first optical fiber for coupling; this step is a receiving and coupling process of the to-be-detected light, and then Step II is performed;

[0054] Step II: the central CPU module generates a digital signal and sends it to the signal acquisition / generation module, sends a driving signal into the local oscillator light generation module, and drives the local oscillator laser to emit a local oscillator light signal of a to-be-detected frequency band; the local oscillator light signal enters the beam splitter through the second optical fiber, and is then divided into a detection signal and a reference signal by the beam splitter; this step is performed simultaneously with Step I as a local oscillator light signal, and then Step III is performed;

[0055] Step III: The detection signal of the local light signal is coupled with the to-be-detected light signal in the first optical fiber path and is received by a high-speed photoelectric detector; the reference signal of the local light signal is transmitted to a calibration photoelectric detector through a beam splitter and an attenuator, the calibration photoelectric detector does not have a fast detection requirement, and after being processed by a signal acquisition / generation module and a central CPU module, a 3f harmonic is extracted, the purpose is to feedback and calibrate the center wavelength of the local light laser, so as to avoid the center wavelength drift of the local light caused by external temperature, voltage drift and other factors; then step IV is performed;

[0056] Step IV: The high-speed photoelectric detector has a fixed bandwidth parameter, for example, 2GHz, at this time, the optical signal passes through the high-speed photoelectric detector to generate a heterodyne signal, the signal is an electrical signal, and the light with a center frequency difference of 2GHz or less from the local light can pass through the high-speed photoelectric detector, the optical signal with a center frequency difference of more than 2GHz is not responded, and the output is in the form of a direct current, and then step V is performed;

[0057] Step V: The heterodyne signal of the electrical signal is screened by a band-pass filter to obtain a signal with the same bandwidth as the band-pass width of the band-pass filter, that is, the channel bandwidth of the spectral scanning channel, and the spectral scanning of the to-be-detected frequency band can be realized by changing the center frequency of the band-pass filter; after being processed by a radio frequency signal processing module, a signal amplified by a radio frequency is obtained after frequency selection;

[0058] The heterodyne signal is an electrical signal at this time, which is processed by a radio frequency signal processing module, the radio frequency signal processing module first removes the direct current through a direct current removal circuit, then screens the signal through a band-pass filter, and the signal passing through the band-pass filter is the selected spectral channel at this time, and the passband bandwidth of the band-pass filter is the spectral resolution; the bandwidth and the center frequency of the band-pass filter can be adjusted. At this time, the center frequency of the band-pass filter is changed, the wavelength scanning function is realized without using the laser tuning mode, and the signal after frequency selection is amplified by a radio frequency and step VI is performed;

[0059] Step VI: The signal amplified by the radio frequency is received by a radio frequency detector, is collected by a signal acquisition / generation module, is fed back to a central CPU module, and a spectral amplitude signal is output after calculation by the central CPU module.

[0060] Figure 3 The step flow of the laser heterodyne detection method adopted by the application is shown. Different from the traditional detection method, the application adopts a high-bandwidth detector to expand the overall wavelength range of the received signal, adopts a software digital filter algorithm to realize a center wavelength variable band-pass filter, and realizes a push-broom process by changing the center wavelength position. The method has obvious advantages, can break away from the push-broom capability limitation of the laser, and realizes the super-spectral resolution capability of the adjustable bandwidth of the full spectral range.

[0061] In the heterodyne technology-based hyperspectral detection method of the application, the implementation of the band-pass filter with variable center frequency can be based on the CPU chip of the center CPU module, specifically, the center CPU module is used to control the peripheral relay circuit to realize the parallel connection of the capacitor and the resistor in the band-pass filter, thereby changing the capacitive reactance and impedance value of the band-pass filter and realizing the change of the center frequency.

[0062] The wavelength scanning range of the heterodyne technology-based hyperspectral detection method of the application can be expanded and is affected by the bandwidth of the detector; when the bandwidth is 2GHz, the scanning wavelength range is 2GHz; when a high-speed detector is used, such as 100-300GHz, the wavelength detection range of the method proposed in the application is 100-300GHz.

[0063] The detection system suitable for the heterodyne technology-based hyperspectral detection method of the application: in the light source part, a local oscillator light generator is used, which is a semiconductor laser with a center wavelength of 700nm, the light source can be tuned with a range of 10pm, and a 5KHz sine wave is used in the design envelope carrier, and a sawtooth wave with a frequency of 10Hz is used for scanning the measured wave peak.

[0064] The telescope system can be self-developed according to actual conditions, and the optical collimation system can select a reflective collimator (RC08FC-P01) of the SORLITE company, and the light wavelength is 0.4-20um.

[0065] The first and second optical fibers (first and second optical fiber combiner) select the (F-CPL-F) series produced by the Newport company of the United States, and the insertion loss is 0.6dB in the wavelength range of 400-1100nm, and the return loss is 55dB.

[0066] The high-speed photoelectric detector selects a customized single-mode detector, and the effective wavelength covers 400-1100nm (hyperspectral detection), and the wavelength covered includes 300-1700nm (hyperspectral detection), the detector response frequency is 300GHz, and the spectral coverage range is 490pm at this time.

[0067] The band-pass filter is a center wavelength variable band-pass filter with a bandwidth of about 6G, and the spectral resolution is 10pm at this time.

[0068] The heterodyne technology-based hyperspectral detection method of the application changes the heterodyne detection method of the laser wavelength non-tunable range from limited to possible, and the implementation method is simple, changes the adjustable hard requirement of changing the center wavelength of the laser into the function implementation of the electronic filter, and the digital control method is more flexible.

[0069] Considering that the laser below 760nm is a non-tunable laser, the wavelength of the local light signal is usually less than 760nm, but not limited thereto. The online hyperspectral detector of the present application, when the central wavelength of the laser is not tunable (such as the wavelength below 760nm), uses the change of the center frequency of the band-pass filter to realize the ultra-high resolution spectral scanning of the laser band below 760nm.

[0070] At present, the visible light tunable range is generally about 10pm, and the wide tunable range visible light semiconductor laser generally has a built-in PZT (piezoelectric ceramic), which is equivalent to the cavity tuning technology, and has high complexity and high cost. The implementation of the hyperspectral detection method of the present application can break the dependence of the heterodyne technology on the tuning range of the laser.

[0071] The online hyperspectral detector of the present application selects a customized single-mode detector for the photodetector, can realize the effective wavelength coverage of 400nm-1100nm for the ultra-high resolution spectral scanning, and can realize the wavelength coverage of 300nm-1700nm for the high resolution spectral scanning. It is worth mentioning that the actual wavelength range that can be covered in actual application is determined by the existing wavelength range that can be covered by all lasers, that is, a certain wavelength exists for the laser (including tunable and non-tunable), and the spectral frequency region within the wavelength range with the bandwidth of the detector is the detectable region.

[0072] Obviously, the above embodiments are only examples for clear illustration, and are not limitations on the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. It is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A hyperspectral detection method based on heterodyne technology, characterized in that, Multi-band spectral scanning can be achieved without relying on a tunable laser as a local oscillator. Applicable detection systems include: a bandpass filter with a variable center frequency and a local oscillator. The bandpass filter is used to perform spectral scanning of the frequency band to be measured by changing the center frequency, and to modify the wavelength width of the spectral window by changing the bandpass width. The local oscillator light generator is used to emit local oscillator light signals in the frequency band to be measured; The hyperspectral detection method includes the following steps: Step 1: Couple the optical signal to be tested with the local oscillator optical signal; Step 2: Process the coupled optical signal into a heterodyne signal of electrical signal; Step 3: Perform spectral scanning of the frequency band to be tested; change the center frequency and bandwidth of the bandpass filter to achieve frequency selection and bandwidth control of the heterodyne signal of the electrical signal, and output the spectral amplitude signal after calculation.

2. The hyperspectral detection method according to claim 1, characterized in that, The applicable detection system specifically includes, in sequence: a telescope system, an optical collimation system, a first optical fiber, a high-speed photodetector, a radio frequency signal processing module, a radio frequency detector, a signal acquisition / generation module, and a central CPU module. The signal acquisition / generation module is also connected in sequence to: a local oscillator generator, a second optical fiber, a beam splitter, an attenuator, and a calibration photodetector; the calibration photodetector is also connected to the signal acquisition / generation module. The radio frequency signal processing module is equipped with a bandpass filter with a variable center frequency; The hyperspectral detection method includes the following steps: Step 1: The optical signal to be measured is received by the telescope system, collimated by the optical collimation system, and then enters the first optical fiber; Step II: The central CPU module generates a digital signal and sends it to the signal acquisition / generation module, which drives the local oscillator optical generator to emit the local oscillator optical signal of the frequency band to be measured. The local oscillator optical signal enters the beam splitter through the second optical fiber and is split into a probe signal and a reference signal by the beam splitter. Step III: The detection signal of the local oscillator optical signal is coupled with the optical signal under test in the first optical fiber and then received by the high-speed photodetector; the reference signal of the local oscillator optical signal is attenuated and then transmitted to the calibration photodetector. After being processed by the signal acquisition / generation module and the central CPU module, the 3f harmonic is extracted and the center wavelength of the local oscillator optical signal is calibrated by feedback. Step IV: The coupled optical signal generates a heterodyne signal after passing through a high-speed photodetector. The light whose frequency differs from the center frequency of the local oscillator light by a value within the fixed bandwidth of the high-speed photodetector passes through the high-speed photodetector. Step V: The heterodyne signal of the electrical signal is filtered through a bandpass filter to select the signal with the same bandwidth as the bandpass width of the bandpass filter, which is the channel bandwidth of the spectral scanning channel. By changing the center frequency of the bandpass filter, the spectral scanning of the frequency band to be measured is realized. After processing by the RF signal processing module, the frequency-selected and RF-amplified signal is obtained. Step VI: The signal amplified by radio frequency is received by the radio frequency detector, acquired by the signal acquisition / generation module, and fed back to the central CPU module. The central CPU module then calculates and outputs the spectral amplitude signal.

3. The hyperspectral detection method according to claim 2, characterized in that, Steps I and II are performed simultaneously.

4. The hyperspectral detection method according to claim 2, characterized in that, In step V, the heterodyne signal is processed by the radio frequency signal processing module, specifically as follows: First, the DC current is eliminated by using a DC blocking circuit; The signal is then filtered using a bandpass filter; Finally, the frequency-selected signal is amplified by radio frequency.

5. The hyperspectral detection method according to any one of claims 1-4, characterized in that, The wavelength of the local oscillator optical signal is less than 760nm.

6. The hyperspectral detection method according to any one of claims 1-4, characterized in that, The spectral resolution is the passband bandwidth of the bandpass filter.

7. The hyperspectral detection method according to any one of claims 1-4, characterized in that, The bandpass filter with a variable center frequency uses a central CPU module to control the peripheral relay circuit, thereby realizing the parallel connection of capacitors and resistors in the bandpass filter, and thus changing the capacitive reactance and impedance values ​​of the bandpass filter to achieve the change of center frequency.

8. An online hyperspectral analyzer, characterized in that, The online hyperspectral analyzer employs the hyperspectral detection method described in any one of claims 1-4 during detection.

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