CN107560738B - A kind of laser beam deviation means for correcting based on dichroic light splitting - Google Patents

Abstract

The present invention proposes a laser beam deflection correction device based on dichroic light splitting. The method mainly includes the following steps: the infrared laser light source and the near-infrared laser light source emit triangular wave signals with the same amplitude and phase, and the infrared laser beam is split through an optical fiber. After the detector, it is divided into two beams, one beam is used as the reference signal I ₁ , and the other beam and the near-infrared laser pass through the fiber coupler and then exit through the collimator. The laser signal I ₂ is obtained by reflecting at the infrared laser detector 401 at a reflection angle of 45 degrees, while the near-infrared laser passes through the spectrometer and is detected by the near-infrared laser detector 402 to obtain the laser signal I ₃ . The deflection relationship eliminates the effect of laser deflection. The invention has simple structure and convenient operation, and has important and extensive application prospects in the field of combustion laser diagnosis.

Description

A Laser Light Deflection Correction Device Based on Dichroic Spectroscopy

(1) Technical field

The invention relates to a laser beam deflection correction device based on dichroic light splitting, in particular to the use of a dichroic light splitter to simultaneously detect test light and calibration light on the same path, and use the calibration light to correct the test light, thereby solving the problem of laser light passing through The light deflection problem caused by refraction in the flame.

(2) Background technology

Effective diagnosis of combustion plays a very important role in various aspects such as industrial safety production, energy saving and emission reduction, scientific research, etc. The design of the chamber can provide an effective reference basis, and the obtained measurement data can also be used to verify and correct the theoretical model of the aero-engine combustion chamber. The measurement methods of combustion parameters can be mainly divided into two types: contact type and non-contact type. The common contact type includes thermocouple temperature measurement method, gas analyzer, etc., and the non-contact type mainly includes acoustic method and spectroscopic method. The response of the contact measurement method is slow, which will affect the flow field. In addition, with the development of combustion technology, the flame temperature is getting higher and higher, so that the contact measurement method can no longer meet the needs of the actual process; the acoustic method is affected by particles in the combustion field, Air flow interference factors have a greater impact and poor anti-interference ability; in contrast, laser diagnostic technology has been widely used due to its advantages of no contact with the measured medium and fast response speed. In recent years, laser-induced fluorescence (LIF, Laser Induced Fluorescence), coherent anti-Stokes Raman scattering spectroscopy (CARS, Coherent Anti-stokes Raman Spectroscopy), laser absorption spectroscopy (LAS, Laser Absorption Spectroscopy) and other optical diagnostic methods The technology is widely used in the measurement of combustion flame temperature, gas composition and other parameters. Among them, laser absorption spectroscopy has the advantages of simple structure, fast measurement speed, high measurement accuracy and high reliability, and has broad application prospects in the field of combustion diagnosis.

The principle of measuring temperature by laser absorption spectroscopy is: use the sawtooth wave signal to modulate the tunable laser diode output laser, so that it sweeps the entire absorption line shape, the laser is first absorbed by specific molecules through the flame, and then reaches the photosensitive surface of the detector to obtain light intensity information. During data processing, the integrated absorption rate is obtained by integrating the entire absorption line shape, and then the temperature and gas concentration are calculated. Therefore, the key to laser absorption spectroscopy is whether the information of light intensity after passing through the flame can be accurately obtained. The flame reaction zone is a high-temperature zone, and its internal components and densities are constantly changing, so the refractive index changes irregularly with time. When using laser absorption spectroscopy to measure the internal parameters of the flame, the laser will have a light deflection effect, resulting in The laser spot that reaches the photosensitive surface of the detector is constantly shaking, and the light intensity signal detected by the detector changes irregularly with time, which seriously affects the measurement results. When measuring a highly pulsating flame such as an aeroengine swirling flame, the density of components inside the flame changes more drastically, resulting in serious changes in the light intensity signal, and even making the measurement impossible.

In order to overcome the deflection effect when the laser passes through the flame, some researchers at home and abroad have proposed a method to make the laser enter the integrating sphere after passing through the flame and then be received by the detector (Almodovar CA, Spearrin RM, Hanson RK.Two-color laser absorption near 5μm for temperature and nitric oxide sensing in high-temperature gases[J].Journal of Quantitative Spectroscopy and Radiative Transfer,2017; H Yang.Tunable diode-laser absorption-based sensors for the detection of water vapor concentration,film thickness and temperature[D]. Duisburg-Essen, 2012; Yang Huinan, Guo Xiaolong, Su Mingxu, Cai Xiaoshu. Online measurement of dynamic thickness of liquid film in the airflow channel based on TDLAS technology [J]. China Laser, 2014,41(12):211-216.). The integrating sphere is a hollow sphere whose inner wall is coated with white diffuse reflection material. The laser light entering the integrating sphere is reflected by the inner wall for many times, and finally evenly distributed on the inner wall, which can ensure that the laser light intensity passing through the integrating sphere and reaching the photosensitive surface of the detector remains constant. Change. However, since the laser light is evenly distributed on the entire inner wall of the sphere after passing through the integrating sphere, the laser signal detected by the detector will become very weak and difficult to detect effectively. In addition, multiple reflections in the integrating sphere are equivalent to lengthening the optical path of the laser , and the laser may also be absorbed in the integrating sphere, which will affect the measurement results. Professor Hansen of Stanford University designed a light deflection non-sensitive light collection system to solve the problem of light deflection (Strand C L. Scanned Wavelength-Modulation Absorption Spectroscopy with Application to Hypersonic Impulse Flow Facilities[D]. Stanford University, 2014; Goldenstein CS, Schultz IA , Strand CL, et al. Hypersonic scramjet testing via TDLAS measurements of temperature and column density in a reflected shock tunnel [C]. Aerospace Sciences Meeting. Maryland, 2014). The basic method is to make the laser beam pass through the flame and be focused into the optical fiber by the focusing lens first, and the laser light reaches the detector through the optical fiber. However, the design of the system is difficult, and the experimental debugging is difficult. In addition, the light intensity loss during the light collection process is large, which is not suitable for industrial applications. Live application. Ritobrata Sur of Stanford University proposed to increase the beam diameter to reduce the influence of light deflection (Sur R, Sun K, Jeffries JB, et al. Scanned-wavelength-modulation-spectroscopy sensor for CO, CO 2, CH 4 and H 2 O in a high-pressure engineering-scale transport-reactor coal gasifier [J]. Fuel, 2015, 150: 102-111.). However, increasing the beam diameter can only ensure that the laser light can still be detected by the detector after the light is deflected. However, due to the uneven distribution of laser light intensity in the cross-section of the beam, laser deflection will still cause the light intensity detected by the detector to jitter, which is not actually the case. The light deflection problem is not solved. Therefore, none of the existing methods can effectively solve the problem of light deflection. In order to effectively measure the internal parameters of the flame and meet the needs of industrial development, a new method that can fundamentally solve the problem of light deflection is urgently needed.

The present invention uses the calibration light to calibrate the test light. First, the signal generator is used to output a triangular wave signal to the laser controller. The laser controller makes the near-infrared tunable laser and the infrared laser emit a triangular wave signal with the same amplitude and phase. The optical fiber beam splitter is divided into two beams, one beam is directly detected by the photodetector 403 without passing through the flame to obtain the reference signal I ₁ ; the other beam and the near-infrared laser pass through the fiber coupler and then exit through the collimator. After the outgoing light passes through the flame, Irradiate the surface of the dichroic beam splitter at 45 degrees, the infrared laser is reflected at a reflection angle of 45 degrees and reaches the infrared detector 401 to obtain the laser signal I ₂ , while the near-infrared laser passes through the dichroic beam splitter and reaches the near-infrared detector 402 to obtain the laser signal Signal I ₃ , so as to realize simultaneous detection of near-infrared laser and infrared laser on the same path. Since the near-infrared laser used in LAS will be absorbed by the combustion products, but the selected infrared laser will not be absorbed, so the infrared laser is only affected by the deflection of the light, and the absorption and deflection relationship between the three laser signals can be used to eliminate the laser deflection effect.

(3) Contents of the invention

The present invention proposes a laser light deflection correction device based on dichroic light splitting, which uses a dichroic light splitter to separate lasers of different wavelengths on the same path, thereby simultaneously detecting infrared lasers and near-infrared lasers passing through flames along the same path, The infrared laser signal is used to correct the near-infrared laser signal to eliminate the influence of light deflection effect on the measurement.

The components used include: signal generator, near-infrared laser controller, infrared laser controller, near-infrared tunable laser source, infrared laser source, fiber coupler, fiber beam splitter, collimator, dichroic beam splitter and photoelectric detector.

The technical scheme adopted in the present invention is: the infrared laser and the near-infrared laser are output with the same triangular wave light intensity signal under the same laser control signal, and the infrared laser is divided into two beams after passing through the optical fiber beam splitter, and one beam is directly burned without passing through the flame. The detector detection is used as a reference signal; the other channel is coupled with the near-infrared laser signal and is incident on the surface of the dichroic beam splitter at a 45-degree incident angle through the flame. The infrared laser is reflected at a 45-degree reflection angle and reaches the infrared laser detector. The laser is detected by the near-infrared laser detector through the spectrometer, so that the infrared laser light intensity signal and the near-infrared laser light intensity signal after passing through the flame are detected simultaneously. Infrared laser will not be absorbed by combustion products, so the influence of light deflection effect can be obtained from the infrared laser signal and reference signal after passing through the flame, and then the near-infrared laser signal can be corrected to eliminate the influence of light deflection.

The advantages of the present invention are: 1. The present invention uses a beam splitter to detect laser signals of different wavelengths on the same path at the same time, which will not weaken the intensity of the laser light reaching the photosensitive surface of the detector, and is convenient for detection; 2. The laser signal passes through two The dichroic beamsplitter is then detected by the detector, and the laser path is not significantly lengthened, which avoids the influence of laser absorption caused by multiple reflections inside the traditional integrating sphere method on the measurement results.

(4) Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a schematic diagram of the optical path of the present invention;

Figure 2 is the infrared laser signal before passing through the flame;

Figure 3 is the near-infrared laser signal before passing through the flame;

Figure 4 is the infrared laser signal after passing through the flame;

Figure 5 is the near-infrared laser signal after passing through the flame;

Figure 6 is the near-infrared laser signal after correction.

Figures

101: near-infrared tunable laser, 102: infrared laser, 201, 202: collimator, 301: dichroic beam splitter, 402: near-infrared laser detector, 401, 402: infrared laser detector.

(5) Specific implementation methods

In Figure 1, the near-infrared laser and the infrared laser are emitted by the collimator 201 after being coupled, and incident on the surface of the dichroic beam splitter 301 at an incident angle of 45 degrees, the reflected light is perpendicular to the incident light and received by the infrared laser detector 401 , the near-infrared laser detector is located behind the dichroic beam splitter and is used to detect the transmitted near-infrared laser. The infrared laser emitted by the collimator 202 is directly received by the detector 403 without passing through the flame as a reference light intensity signal.

Step 1: The signal generator outputs the same triangular wave signal to the near-infrared laser controller and the infrared laser controller, so that the near-infrared laser and the infrared laser emit triangular wave signals with the same amplitude and phase;

Step 2: The infrared laser is divided into two beams through the optical fiber beam splitter. One beam and the near-infrared laser pass through the fiber coupler and then exit through the collimator 201, and the other directly exits through the collimator 202 and reaches the detector 403 to obtain The infrared laser signal (seeing Fig. 2) is recorded as I ₁ as the reference light intensity signal;

Step 3: The laser light emitted by the collimator 201 is incident on the surface of the dichroic beam splitter at an incident angle of 45 degrees through the flame, the infrared laser is reflected at the infrared laser detector 401 at a reflection angle of 45 degrees, and the near-infrared laser passes through the The light sheet reaches the near-infrared laser detector 402, so that the infrared laser light intensity signal (see Figure 4) and the near-infrared laser light intensity signal (see Figure 5) after passing through the flame are obtained at the same time, which are respectively denoted as I ₂ and I ₃ ;

Step 4: Normalize the reference light signal, the infrared laser signal after passing through the flame, and the near-infrared laser signal using formula (1):

Wherein, I _i (i=1, 2 or 3) is the original laser signal, I _imin and I _imax are the minimum and maximum values of the original laser signal, respectively, and I _i1 is the normalized laser signal.

Step 5: The light intensity fluctuation ΔI caused by the deflection effect of the laser on the signal can be obtained by formula (2):

Δ=I ₂₁ -I ₁₁ (2)

Wherein, I ₁₁ and I ₂₁ are the normalized infrared laser signals without flame and with flame, respectively.

Step 6: The near-infrared laser signal I after correction can be obtained by the following formula:

I=I ₃₁ -ΔI (2)

Fig. 6 is the near-infrared laser signal after correction, which shows that the present invention can overcome the influence brought by the deflection effect of the flame, and then use the principle of LAS to accurately measure the flame parameters.

Claims (3)

1. A laser light deflection correction device based on dichroic light splitting, including a signal generator, a near-infrared laser controller, an infrared laser controller, a near-infrared tunable laser light source, an infrared laser light source, an optical fiber coupler, an optical fiber splitter Beamer, collimator, dichroic beam splitter and photodetector, characterized in that the infrared laser is divided into two beams through the optical fiber beam splitter, one beam is used as a reference, and the other beam and the near-infrared laser pass through the fiber coupler and then pass through The collimator exits, and the outgoing light passes through the flame under test and irradiates the dichroic beam splitter. The infrared laser detector is placed on the path of the reflected light, and the near-infrared laser detector is placed on the path of the transmitted light. The relationship between absorption and deflection eliminates the influence of laser deflection. 2. A kind of laser beam deflection correction device based on dichroic light separation according to claim 1, characterized in that, the infrared laser light source and the near-infrared tunable laser light source emit triangular wave signals with the same amplitude and phase, and the infrared laser light source The fiber beam splitter is divided into two beams, one beam is directly detected by the photodetector (403) without passing through the flame to obtain the reference signal I ₁ ; the other beam is coupled with the near-infrared laser, passes through the flame, and then irradiates the two beams at an incident angle of 45 degrees. On the chromatic beam splitter, the infrared laser is reflected at a reflection angle of 45 degrees and reaches the infrared detector (401) to obtain a laser signal I ₂ , and the near-infrared laser passes through the beam splitter and reaches the near-infrared detector (402) to obtain a laser signal I ₃ . 3. A kind of laser light deflection correction device based on dichroic light separation according to claim 2, characterized in that, the infrared laser light passes through the flame without being absorbed, and the light intensity signal is only affected by the light deflection effect, Therefore, the light intensity fluctuation ΔI caused by the deflection effect produced by the flame on the signal is formula (1): ΔI=I ₂₁ -I ₁₁ (1) Among them, I ₁₁ and I ₂₁ are the infrared laser signals without flame and flame after normalization respectively, and the normalized laser signal is obtained by formula (2): Among them, I _i (i=1, 2 or 3) is the original laser signal, I _imin and I _imax are the minimum and maximum values of the original laser signal respectively, I _i1 is the normalized laser signal, and the corrected near Infrared laser signal I can be obtained by formula (3): I=I ₃₁ -ΔI (3) Among them, I ₃₁ is the normalized near-infrared laser signal. The corrected near-infrared laser signal overcomes the laser deflection effect, and can use the principle of laser absorption method to accurately measure the flame parameters.