Portable Temperature Sensing Device Based on On-Chip Optical Microcavities
Objective Optical sensors offer significant advantage over traditional electrical sensors,particularly in their immunity to electromagnetic interference.Whispering gallery mode(WGM)microresonator sensors,a typical optical sensor type,possess an ultra-high quality(Q)factor and minimal mode volume,which amplify light-matter interactions and greatly enhance sensor sensitivity.As environment conditions change,the spectral properties of the WGM resonator—such as frequency shifts,mode splitting,and linewidth broadening—also change.Utilizing this mechanism,WGM resonators have been deployed in various applications,including angular velocity sensing,optical routing,nanoparticle detection,and atomic ion detection.However,current applications typically require large-scale,specialized laboratory equipment,which hinders the practical use of WGM microresonators outside laboratory settings.Two primary obstacles limit the practical application of WGM microresonators:1)ensuring stable coupling of the input laser into the on-chip optical microcavity,and 2)integrating large laboratory equipment like tunable lasers,oscilloscopes,waveform generators,and control computers into a portable device.To address these challenges,we propose a portable temperature sensing device using a WGM on-chip microresonator sensing chip.This device integrates functional modules such as a tunable laser,laser driver,oscilloscope,waveform generator,photodetector,and on-chip optical microcavity temperature sensor,along with dedicated software for data monitoring and storage.This enables high-precision,wide-range temperature measurement outside laboratory environments,demonstrating the potential of WGM optical sensors for practical applications and serving as a model for portable on-chip microcavity sensing.Methods The portable temperature sensing device comprises four main components:the optical subsystem,driver and control circuits,control and processing circuits,and host computer software.The primary functions are achieved through the driver board and control board(Fig.1),both using the STM32F103 as the main controller with serial port communication.The driver board circuit includes three modules(Fig.2):a current feedback control circuit for the distributed feedback laser(DFB),a TEC temperature control circuit for the DFB laser temperature,and another TEC temperature control circuit for the on-chip optical micro-ring temperature.The control board circuit has four modules(Fig.3):a DAC module for triangular wave signal generation to tune the DFB output laser wavelength,an ADC module for capturing mode waveforms,an LCD touchscreen for human-machine interaction,and a Wi-Fi module for communication with the host computer.During operation,the main controller on the control board generates a digital triangular wave signal to control the DAC module,which outputs a triangular wave analog signal to tune the laser wavelength.This enables the DFB laser to perform continuous wavelength periodic scanning of the optical microcavity.The laser enters the optical microcavity,resonates within,and exits to the photodetector,where the transmission spectrum signal is converted to an electrical signal.The signal is collected and digitized by the ADC module.The main controller filters and locks onto the resonant mode,calculating the current temperature based on the resonant mode position,and then displaying it on the LCD.In addition,sensor data can be transmitted to host computer software via LAN or Alibaba Cloud for real-time monitoring and abnormal data storage through the Wi-Fi module.The driver board provides DC bias current to the DFB laser,continuously monitors DFB laser and microcavity temperatures,and maintains target temperature using a PID algorithm.It also handles command queries and settings from the control board,returning corresponding results.For portability,the micro-ring resonator and input/output fibers are end-coupled,achieving 40％coupling efficiency after packaging.A 3D-printed casing encloses the submodules(Fig.4).Results and Discussions The current feedback control circuit and two TEC temperature control circuits on the driver board are initially tested for performance and stability(Fig.8).The results indicate that the current feedback circuit provides high output current accuracy with minimal fluctuation,allowing precise control of laser injection current and stable wavelength operation.The laser's temperature control circuit maintains a constant operating temperature,stabilizing output wavelengths,while the WGM resonator's TEC temperature control circuit precisely regulates its temperature,supporting accurate temperature measurement experiments.Subsequently,the portable device's temperature measurement performance is tested(Fig.9).During a triangular wave frequency sweep cycle,the micro-ring temperature is controlled from 20 ℃ to 31 ℃.The resonant mode transmission spectra are collected,showing a redshift in the resonant mode position with rising micro-ring temperature.With DFB laser current controlled within 180-280 mA,and varying temperature conditions,the on-chip micro-ring resonator's temperature varies within 20 ℃ to 38 ℃.The resonant mode waveforms are collected,showing a linear relationship between mode position and temperature,with repeatability across measurements.The maximum temperature range of the device,determined by the laser's wavelength tunability of 0.34 nm,is 17 ℃.with an average measurement error of 0.045 ℃,a temperature sensitivity of 0.02 nm/℃,and a resolution limit of approximately 0.009 ℃.Conclusions A portable temperature sensing device based on an on-chip optical micro-ring resonator has been designed.The device employs a Si3N4 on-chip micro-ring resonator as the temperature sensor,featuring a DFB driver board,control board,and power supply circuit.The driver board powers and tunes the laser,which is coupled to the micro-ring.Temperature measurement relies on the resonant wavelength's linear temperature dependence.The control board automates resonance waveform acquisition,mode locking,and temperature display,with dedicated software for data reception and monitoring.Comparisons with standard laboratory instruments demonstrate that the device's driving circuit exhibits high stability and precision.Temperature sensing experiments further confirm that the device provides high accuracy and repeatability,making it a viable substitute for traditional laboratory equipment for temperature measurements.The on-chip microcavity-based mode-shifting sensing mechanism can be applied to detect various nanoscale environmental parameters,such as temperature,magnetic fields,gases,stress,and acoustic waves,laying a crucial foundation for practical applications of on-chip microcavity sensors.
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