Transformer Fault Characteristic Gas Optical Feedback Frequency Locking Asymmetric Linear F-P Cavity Enhanced Raman Spectroscopy Detection
The oil-immersed power transformer represents a pivotal component within the framework of a power system.The accurate detection of power transformer operation status and the issuing of fault warnings are of great significance in ensuring the safe and reliable operation of the power grid.The composition and content of dissolved gases in transformer oil are closely related to the fault state of the transformer.One of the most effective methods for diagnosing the operational state of a transformer is the accurate detection and analysis of the characteristic gases associated with transformer faults.Trace gas sensing based on laser spectroscopy plays a pivotal role in numerous fields,including environmental monitoring,urban surveillance,industrial process control,medical diagnostics,and agronomy.Raman spectroscopy is based on the Raman scattering effect of matter,which can be used to detect all gases except single atom gases.The simultaneous qualitative and quantitative analysis of multi-component gases can be achieved by utilizing a single wavelength laser.Nevertheless,the extremely low Raman scattering cross-section of gas represents a significant limitation in the detection sensitivity of Raman spectroscopy,which in turn constrains its broad applicability in the field of trace gas detection and analysis.Fiber-Enhanced Raman Spectroscopy(FERS)is a Raman scattering signal enhancement technology based on hollow fibers.Its objective is to improve the collection efficiency of Raman scattered light.However,the balance time required for gas to enter the fiber is too long.Cavity-Enhanced Raman Spectroscopy(CERS)employs the linear correlation between the Raman signal and laser power to enhance the Raman signal of gases and improve the sensitivity of gas detection by stabilizing the laser to a linear Fabry-Pérot(F-P)cavity,thereby enabling the accumulation of optical power.One of the key issues associated with F-P cavity enhancement technology based on optical feedback frequency locking is the potential for the direct reflection light of the resonator input mirror to interfere with the laser frequency locking,thereby preventing the accumulation of power within the cavity.Consequently,it is imperative to attenuate the intensity of the direct reflection light during the optical feedback frequency locking process.At present,the optical feedback frequency locking technology must forego the simplicity and sensitivity of the Cavity-Enhanced Raman Spectroscopy(CERS)gas sensing system in order to circumvent the direct reflection of the resonant cavity,which would otherwise compromise the frequency locking and power accumulation.This paper proposes a simple and highly sensitive optical feedback frequency locking asymmetric cavity mirror linear F-P cavity enhanced Raman spectroscopy gas detection technology.The phase relationship between resonant light and direct reflection light is analyzed.The influence of attenuated direct reflection on optical feedback frequency locking is analyzed using the cavity reflected light field function.The theoretical analysis of the influence of direct reflection and the resonant light feedback coefficient on the intensity of the reflected light field is presented.A novel asymmetric mirror linear F-P cavity direct reflection attenuation model is proposed.The results of the simulation demonstrate that the optical feedback frequency locking range gradually increases with a reduction in the reflectivity of the cavity input mirror.The theoretical feasibility of achieving optical feedback frequency locking in a linear F-P cavity by reducing the reflectivity of the front mirror in order to attenuate direct reflection is demonstrated.On this basis,an optical feedback frequency locking asymmetric cavity mirror linear F-P cavity enhanced Raman spectroscopy detection platform was designed and constructed.In order to verify the feasibility of the asymmetric cavity mirror linear F-P cavity direct reflection attenuation model,frequency locking experiments were performed using cavity input mirrors with different reflectivity.By regulating the reflectivity of the cavity input mirror to be significantly lower than that of the cavity output mirror,the resonant light is dominant in the reflected light,and the optical feedback frequency locking is successfully achieved.Finally,a plane mirror with a reflectivity of 99.96%and a flat concave mirror with a reflectivity of 99.994%were selected as the front and rear mirrors of the linear F-P cavity,respectively.The laser was able to establish a stable basic transverse mode TEM00 power accumulation within the cavity.When the input power is 80 mW,the laser power in the cavity is approximately 320 W,and the gain multiple is 4 000 times.The backward Raman scattering light collection method was employed.In the context of a gas chamber pressure of 0.1 MPa and an integration time of 60 s,the detection limits of the optical feedback frequency-locked asymmetric cavity mirror linear F-P cavity enhanced Raman spectroscopy detection platform for the main fault characteristic gases CH4,C2H6,C2H4,C2H2,CO and CO2 of the transformer were 1.3,4.4,2.7,1.2,10.5 and 5.1 μL/L,respectively.The optical feedback frequency locking asymmetric cavity mirror linear F-P cavity enhanced gas Raman sensing system exhibits high detection sensitivity and high spectral resolution,rendering it a promising candidate for transformer fault characteristic gas detection.A more sensitive detection limit can be achieved by utilizing a higher power diode laser,a higher reflectivity cavity mirror and an increased integration time.The asymmetric cavity mirrors direct reflection attenuation model proposed in this paper can also be employed to lock the frequency of other types of diode lasers to a linear F-P cavity.The technology of cavity-enhanced spectroscopy has the potential to be of significant value in a number of applications.
Linear F-P cavityOptical feedbackAsymmetric cavity mirrorRaman spectrumGas detection
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