Whispering-gallery-mode optical micro-cavities have been extensively researched in narrow linewidth filters, biosensors, and nonlinear optics due to their small mode volume and ultra-high Q factor. Data acquisition and identification of mode spectral lines in real time are critical for experimental process control and index detection in research on optical micro-cavities. There are two typical detection strategies for spectral signals of optical micro-cavities. The first scheme uses a broadband light source to excite the whispering gallery modes of the optical micro-cavities, and the spectral signals are detected and evaluated by a spectrometer. It is costly, however, and thus this approach is only suitable for optical micro-cavities with a Q factor of less than 106. The other method for detecting spectral signals is to stimulate the whispering gallery modes by scanning the optical micro-cavity with a narrow linewidth laser. The stimulated spectral signal is then transformed by a photoelectric detector and sent to an oscilloscope for analysis. This approach is appropriate for ultra-high-Q optical micro-cavities, but it is incapable of analyzing the spectral signal in real time. In addition, there is an urgent need for an acquisition system with a high sampling rate, a rapid reaction time, and the capacity to monitor and analyze spectral data in real time for applications in high-sensitivity sensing of micro-cavities. Hence, a spectral signal acquisition system for optical micro-cavities is explored and constructed in this study to solve these issues. The system consists of an acquisition module, a data storage module, a data analysis module, and a data display module, which can show and analyze the spectral signals of optical micro-cavities in real time and has the benefit of quick acquisition. Moreover, it can set an arbitrary sampling interval and sampling time according to the requirements of different sensing systems, which solves the problems of capturing, analyzing, and displaying spectral signals in optical micro-cavities.
This signal acquisition system is separated into acquisition and software modules (Fig. 1). The analog signal is amplified and filtered by the conditioning circuit in the acquisition module (Fig. 2) before being transformed into a digital signal by the analog-to-digital converter (ADC). The transformed data stream is then transmitted to the processor for signal processing before the peripheral component interface (PCI) communicates with the upper computer software for data transmission, and the display operation is finished in the upper computer software. The acquisition system's software module is separated into five sections: trigger edge setting, system configuration, acquisition configuration, main menu, and data display. The trigger acquisition operation can be specified as a rising or trailing edge of the waveform in the trigger edge setting, and the rising edge trigger is valid at the positive edge. The time interval, sampling frequency, trigger frequency, and reference voltage can all be adjusted flexibly in the system configuration. After the parameters are set, they could be locked by the parameter lock feature in the system configuration to ensure the smooth execution of the acquisition procedure. The function of the acquisition configuration section is primarily to finish the setup of arbitrary parameters necessary for the number of acquisitions and the average number of acquisition experiments and control the acquisition process. The procedure of initiating and terminating the acquisition is completed via the main menu. The data display section shows the real-time waveform that is delivered to the upper computer software via NI Measurement Studio control. The development and design of the upper computer software for the acquisition system make use of Visual Studio software, which is equipped with the NI Measurement Studio integrated suite and is written in the C# programming language. The integrated suite provides a set of tightly integrated. NET controls for the Microsoft Visual Studio.NET environment, which allows the user to build virtual instrumentation systems.
First, the fundamental function of the spectral signal acquisition system is validated (Fig. 5), and the system's feasible basic acquisition function is proven. Then, the stability of the acquisition system is examined (Fig. 6), and the experimental findings reveal that the acquisition system fully fulfills the stability criteria of optical micro-acquisition cavities. After that, a fiber microsphere cavity created by arc discharge (Fig. 8) and a silica crystalline micro-disk cavity prepared by ultra-precision polishing (Fig. 9) are examined separately. The transmission spectra of the fiber microsphere cavity and the silica crystal micro-disk cavity are measured and traced, with the Q factor of the former being 2.26×106 and that of the latter being 109. The acquisition system can excellently suppress noise and maintain steady mode spectral lines over extended periods. The optical micro-cavity system with an ultra-high Q factor has high reliability and could be utilized to create future micro-cavity sensing applications.
A spectral signal acquisition system is designed for experimental systems of optical micro-cavities with an ultra-high Q factor. First, the triangle wave signal generated by the arbitrary function generator is acquired to evaluate the performance of this spectral acquisition system. It is demonstrated that this acquisition system can perform the fundamental acquisition function and has long-term stability. Second, this system is used to examine the spectral signal of a high-Q fiber microsphere cavity. The results indicate that this system is capable of realizing the flexible configuration of sampling of the micro-cavity system. Finally, the denoising feature of the ultra-high-Q silica crystal micro-disk cavity is shown by this system. The results show that the developed spectral signal acquisition system has the advantages of high reliability, a high sampling rate, and fast response, and it can acquire the spectral signals of ultra-high-Q micro-cavities and analyze the modes in real time. It provides great convenience for spectrum acquisition in micro-cavity research and can greatly broaden the potential applications of optical micro-cavity devices.
