Laser methane gas sensors have been increasingly accepted in coal mine safety monitoring. Most laser spectroscopic methane gas sensors are based in BFB lasers at around 1650nm. However, they suffer from high power consumption and high cost due to temperature control is required for laser diode operation at constant temperature. VCSEL lasers have offered low operation current and low power consumption when operating at non-TEC mode. However, it is found that the interference noise is critical for laser methane detection. This paper report typical results of the laser diode ripple characterization method and methods of noise reduction methods are discussed

Dong, Y.

Grattan, K. T. V.

Jin, G.

Kong, L.

Liu, T.

Meng, H.

Sun, T.

Wang, J.

English

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Research on VCSEL interference analysis and elimination method

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              City, University of London Institutional RepositoryCitation: Kong, L., Jin, G., Meng, H., Dong, Y., Wang, J., Liu, T., Sun, T. ORCID: 0000-0003-3861-8933 and Grattan, K. T. V. ORCID: 0000-0003-2250-3832 (2019). Research on VCSEL interference analysis and elimination method. Proceedings of SPIE - The International Society for Optical Engineering, 11340, 113401W. doi: 10.1117/12.2548147 This is the accepted version of the paper. This version of the publication may differ from the final published version. Permanent repository link:  https://openaccess.city.ac.uk/id/eprint/23868/Link to published version: http://dx.doi.org/10.1117/12.2548147Copyright and reuse: City Research Online aims to make research outputs of City, University of London available to a wider audience. Copyright and Moral Rights remain with the author(s) and/or copyright holders. URLs from City Research Online may be freely distributed and linked to.City Research Online:            http://openaccess.city.ac.uk/            publications@city.ac.ukCity Research OnlineResearch on VCSEL Interference Analysis and Elimination MethodKong Lingyu1,2， Jin Guangxian²，Meng Hui²，Dong Yuanyuan²，Wang Jingyu²，Liu Tongyu1,2*，Sun Tong3，and Kenneth T. V. GRATTAN1,3(1Laser Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan250014,China;2Shandong Micro-sensor Photonics Co. Ltd, Jinan, China 250103;3School of Mathematics, Computer Science & Engineering, City, University of London, London,EC1V 0HB, UKAbstractLaser methane gas sensors have been increasingly accepted in coal mine safety monitoring. Most laser spectroscopicmethane gas sensors are based in BFB lasers at around 1650nm. However, they suffer from high power consumption andhigh cost due to temperature control is required for laser diode operation at constant temperature. VCSEL lasers haveoffered low operation current and low power consumption when operating at non-TEC mode. However, it is found thatthe interference noise is critical for laser methane detection. This paper report typical results of the laser diode ripplecharacterization method and methods of noise reduction methods are discussed..Keywords：VCSEL，laser diode，gas detection，ripple，interference1 INTRODUCTIONLaser methane modules are used in major industrial and mining enterprises due to their low power and high detectionaccuracy. When the laser methane module measures gas, there are interference problems due to the device problem orother factors used, the main stability data indicators such as ripple are unreliable, and the gas absorption peak image isslightly deformed, so it cannot be used when the gas content is small. Observing the absorption peak image through thesoftware in time, affecting the measurement accuracy, and completely eliminating the interference is difficult to achieve,so the interference must be controlled within a small range.The VCSEL laser is used in the laser methane module. The VCSEL laser is a vertical cavity surface emitting laser. It isdifferent from LED and LD. It is based on InP semiconductor materials. The utility model has the advantages of smallvolume, circular spot output, single longitudinal mode output, small threshold current, low price and easy integration intoa large area array. The laser methane module is applied to TDLAS technology, and VCSEL can be applied to the partwith higher power consumption limit in TDLAS-based gas detection.*Funding support: Major science and technology innovation project of Shandong Province China, (2018CXGC0607)E-mail：tongyu.liu@vip.iss-ms.com, phone：+086 13864169129.1AOPC 2019: Optical Fiber Sensors and Communication, edited by Jie Zhang,Songnian Fu, Jun Yang, Proc. of SPIE Vol. 11340, 113401W · © 2019 SPIECCC code: 0277-786X/19/$21 · doi: 10.1117/12.2548147Proc. of SPIE Vol. 11340  113401W-1In the experimental stage of the laser methane module, the connection test software observed its various stability dataindicators, but the data showed that the ripple results were not very satisfactory. Therefore, the data is derived, the fittingcalculation is performed, and the interference intensity is calculated. The maximum interference intensity was calculatedto be four parts of ten thousandths by six-th polynomial fittings and two-order fittings. In the experimental analysis stage,the cause of the interference is first analyzed. Through experimental and mathematical methods, such as using theformula to calculate the cavity length generated by the interference, correspondingly find the position where theinterference may occur and the device or other problems. Ripple elimination experiments were also performed as needed.In summary, the problem of ripple generation is detected experimentally and the ripple is minimized to improve theaccuracy and reliability of the laser methane module during gas detection.2 BASIC PRINCIPLE OF INTERFERENCE FORMATIONThe laser methane module is mainly composed of a laser, a photodetector and the like. The light emitted by the laserpasses through the mirror in the optical path and finally reaches the photodetector. During this period, stray light orreflection is generated to form multi-beam interference, and the partially reflected beam returns along the original pathand encounters interference [1].In the usual experiment we got:1) When there is no methane concentration, the interference intensity is <0.4‰, but the curve after normalization is in atwisted state; 2) When 1.5% methane gas was introduced, the normalized curve clearly observed the baseline distortion;the interference curve and the normalized curve were significantly different when no methane gas was introduced. Theinterference curve is obtained by six fitting methods, and the normalized curve is obtained by a quadratic fitting method.The above differences occur because the fitting methods used are different. Because of the phenomenon of baselinedistortion and other phenomena, the measurement accuracy is not high, and there may be a small amount of gas presentat the baseline distortion.3 related experiments to the problem of interferenceIn order to verify the possible problem of interference in the laser methane module optical path, suppose the problempoint1) between the laser component and the PD component; 2) Inside the laser component:3) Reflection caused by thelaser component/PD component. The distance between the laser component and the PD component is 68.94 mm, thedistance from the laser component chip to the lens is 5 mm, the distance from the laser oblique window to the lens is 3.53mm, and the distance from the PD component ball to the PD chip is 0.5 mm.3.1 Experimental principleThe interference signal is related to the cavity length L. By measuring the change of the interference signal, the change ofthe cavity length L can be derived, corresponding to the module, thereby obtaining a problem point caused by theinterference [4].Calculate the basic formula for the length of the interference cavity from the interference pattern: 1221-21L（1）where λ1 andλ2 are the starting and the ending wavelength, and L is the interference cavity length.Proc. of SPIE Vol. 11340  113401W-2to the wavelength information and curve information displayed by the module, and finds the problem caused by theinterference according to the optical path diagram. The sixth-order polynomial fitting curve of the light source and thequadratic fitting signal of the probe were observed by software to see the interference fringes in the ripple. The ripple ofthe interference fringes is calculated by the raw data collected by the software at the starting wavelength λ1 of one of theinterference periods and the ending wavelength λ2. Calculate the cavity length resulting from the interference.3.3 Experimental data collectionFigure 1 Light source interference signalFigure 2 Light source interference signal 23.2 Experimental contentThe module link PC reads the original signal and the real-time curve, calculates the interference cavity length accordingProc. of SPIE Vol. 11340  113401W-3Figure 3 Light source interference signal 33.4Experimental data integrationTable 1 Source interference signal data integrationvariableLight source interference signal1 2 3Wavelength range（nm） 1.091 1.299 1.119Starting wavelength（nm） 1647.606 1647.496 1647.583Termination wavelength（nm） 1648.697 1648.795 1648.709Scan points 600 600 600Number of peaks 66 8 3Center wavelength（nm） 1648.1515 1648.1455 1648.146Wavelength range（min） 0.991 1.199 1.019Wavelength range（max） 1.191 1.399 1.219Wavelength stepping（nm/point） 0.001818333 0.002165 0.001865Crest spacing（nm） 0.016530303 0.162375 0.373Crest spacing（min） 0.015015152 0.149875 0.339666667Crest spacing（max） 0.018045455 0.174875 0.406333333Cavity length（L） 82.16435482 8.364537611 3.641267074Cavity length（Lmax） 90.4554098 9.062163767 3.998604373Cavity length（Lmin 75.26558447 7.766643572 3.342557716Proc. of SPIE Vol. 11340  113401W-4The length of the cavity in the table is calculated according to the formula, and other data are sorted out by the collecteddata.3.5 Experimental conclusionsBy observing that the interference of three cycles is obvious, the interference cavity length is calculated.1) Light source interference signal 1 - long distance interference: interference generated between the laser componentand the PD component;2) light source interference signal 2 - short distance interference: interference generated inside the laser assembly;3) Light source interference signal 3 - Ultra short distance interference: interference generated inside the laser component/ inside the PD.4 Ripple elimination experiments4.1 Experimental purposes and methods. In order to eliminate the ripple, the module with the ripple failure isblackened by the surface of the PD lens, mainly to attenuate the reflection intensity and eliminate the reflected light inthe optical path, thereby reducing the ripple of the system [2].4.2 Experimental contentThe GJGX100M intrinsically safe laser methane software (20180201) and Laser ripple test software V1.0 were used inthe experiment. The data was obtained through these two software, and the data was exported. After six fittings, thesystem ripple was controlled within 0.5‰. At the same time, there will be no obvious periodic regular interference on thewaveform. This is an expectation of the experiment.In the packaged module, the laser sleeve has been fixed and cannot be manipulated, and the interference inside the PDcannot be interfered. Therefore, the most convenient operation is the interference between the laser lens and the PD lens,which can be blackened on the PD lens. This way weakens the reflected light, which in turn reduces the ripple of thesystem.Black marks are used in turn and black 704 silica gel is used to black spots at different positions of the PD lens to see theeffect of ripple removal.4.3Experimental dataTable 2 Ripple elimination experimental dataSerialnumberBefore blacking After blackingTest the next day afterblackeningRipple Gain Ripple Gain Ripple Gain1 0.0008 130 0.0003 104 0.0004 1062 0.0011 130 0.0005 142 0.0007 1013 0.0004 4 0.0004 42 / /4 0.0013 9 0.0004 7 0.0006 75 0.0004 21 0.0003 35 / /6 0.0006 56 0.0008 78 / /7 0.0004 14 / /8 0.0002 22 / /9 0.0003 10 / /Proc. of SPIE Vol. 11340  113401W-51) In the sample with the elimination effect, the position of the black spot is best applied to the center of the PD incidentlight spot.2) The elimination effect is also related to the gain of the module (ie, the light intensity), and the module with low gain(light power) eliminates the effect.5 ConclusionThe exiting light intensity of the laser causes interference, and the emitted light of the laser is reflected in the optical path,and then reflected back and then satisfies the interference condition together with the outgoing light of the laser to forminterference, and finally the measurement accuracy is not high. Each device in the optical path forms interference atdifferent distances. At present, the problem of interference is initially found through experimental methods. The next stepis to eliminate interference by software method and further improve the measurement accuracy of the module.References[1] Jiang Xiaofeng, Lin Chun, Xie Haihe etc. MEMS F-P interferometry pressure sensor [J] Infrared and LaserEngineering, 2014,43(7): 2257-2262.[2]Li Shuang, Qiu Zhenwei, Wang Xiangjing. Stray light simulation and analysis of space-borne spatial heterodynespectrometer for monitoring greenhouse gases [J]. Infrared and Laser Engineering, 2015,44(2): 616-619.[3]Liu Tongyu, Wang Jiqiang, et al, Optical fiber sensor for mine safety Internet of things, Industry and MineAutomation,2018.[4]Han Naiqian, Xie Linzhen, Xu Anshi, A New Method of the Length Measurement of FP Cavity, Optical Technology,19984.4 Experimental conclusionsProc. of SPIE Vol. 11340  113401W-6

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Research on VCSEL interference analysis and elimination method

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