The Raman Spectroscopy Signal Detection Based On Digital Lock-In Amplifier

Raman spectroscopy is a mature technology that has experienced decades of development. Relies on its own unique advantages, it has been widely applied in the field of substance identification and molecular structure study. In fact Raman spectroscopy signals are usually very weak, common methods often cannot achieve the detection sensitivity requirements. In this issue, scientists have invented a series of techniques to enhance Raman spectroscopy, such as Raman spectroscopy resonance technology and surface-enhanced Raman spectroscopy. These methods are effective in enhancing the strength of the Raman spectroscopy signal and improving the detection sensitivity as well. To increase signal intensity in the Raman spectroscopy has been successful, but direct detection of weak Raman spectroscopy signals has rarely been attempted. With the development of weak signal detection theory and related technology, the detection sensitivity has been greatly improved, which provides a new way to improve the detection sensitivity of Raman spectroscopy signal.Lock-in amplifier is a representative weak signal detection product. It bases on the correlation detection theory, using the reference signal and the measured signal are strong correlation but the reference signal and the noise signal are weak correlation characteristics to detect the signal from lots of background noise. Initial lock-in amplifier is composed of analog devices, but analog devices are often inherent drift and poor tolerance defect. With the increasing popularity and application of digital signal processing technology and high-performance digital signal processing hardware, the lock-in amplifier algorithms can take advantage of digital signal processing technology to complete, digital lock-in amplifier gradually replaced the traditional analog lock-in amplifier and become mainstream.Considering the Raman spectra and lock-in amplifier technology’s needs and technical characteristics, thesis uses the digital signal processing hardware designed a digital lock-in amplifier system, and applied the system to the Raman spectroscopy signal detection to improve the accuracy of Raman spectroscopy signal detection.Thesis firstly introduces the relevant background knowledge of Raman spectroscopy and lock-in amplifier, and then proposes a scheme that uses the lock-in amplifier to solve the Raman spectroscopy signal detection problem. Thesis has designed a lock-in amplifier system based on the dsPIC33FJ256GP710 DSC, the introduction of hardware divided into four sections:excitation signal generating module, analog circuit modules, digital system module and PC module design, the software design divided into two parts:DSC program design and PC program design, detailing the lock-in amplifier core algorithm and C# data received program design.The thesis experiment is divided into two parts. Firstly thesis designs a confirmatory experiment that used the Wheatstone bridge in the classic model of stress measurement. It has three equal value resistors, and then destroys the bridge balance by changing the fourth resistor’s value.After that the Wheatstone bridge will output the voltage signal. We use the digital lock-in amplifier for this signal measurement, and then get the resistance value. The experimental results accurately obtain the bridge output signal, thus validating the feasibility of the design of digital lock-in amplifier hardware architecture and software algorithms. The second experiment is the core experiment. At this stage of the experiment the main task is apply the digital lock-in amplifier to the Raman spectra acquisition system. Thesis set up a simple Raman optical path and use a chopper to obtain a reference signal for digital lock-in amplifier, using the PMT to obtain sample NaSO4’s Raman spectrum signal. After processing the signal will be detected by the digital lock-in amplifier, then the lock-in amplifier will get the final results by algorithm processing. Because the signal output is associated with the PMT supply voltage, thesis designs several other experiments, and then gets several sets of data. After data processing, the final conclusions in line with expectations. The experiment proves this design of digital lock-in amplifier is suitable for Raman spectroscopic detection.