Characterization of a micro-ring resonator based photoacoustic microscopy system

This study investigates the development and optimization of a novel photoacoustic microscopy (PAM) system incorporating a micro-ring resonator (MRR) for advanced ultrasound detection. PAM is a powerful imaging technique recognized for its ability to visualize optical absorption contrast in biological tissues without the need for contrast agents, enabling high-resolution imaging with lateral resolution determined solely by the optical diffraction limit. The primary aim of this research was to construct and refine a device capable of capturing photoacoustic signals generated by a pulsed laser and analyzing these signals to detect the Doppler effect in fluids, potentially facilitating non-invasive measurements of blood flow velocities within complex vascular networks. Initial tests confirmed the device’s capability to generate and detect high-quality photoacoustic images. Key modifications were implemented to enhance the device’s performance: a variable neutral density filter was added in the beam path to allow precise control of laser power, a beam block was introduced to absorb excess energy reflected from the filter, and a 3-axis stage was used to couple the excitation beam to an optical fiber, achieving a coupling efficiency of 60%. Additionally, a collimator was introduced to stabilize and direct the laser beam more effectively, and the alignment of the photodetector was optimized to maximize signal-to-noise ratio. Characterization of the device revealed several critical performance metrics: the Q factor, which indicates the resonator’s sensitivity, was found to be lower than expected, potentially due to manufacturing limitations. Nonetheless, the lateral resolution was determined to be 5.5 µm and the axial resolution was measured at 12 µm, offering promising precision for various applications. With a field of view of 0.8 mm, the device proved well-suited for small to medium-sized samples, presenting exciting potential for targeted investigations, though larger specimens may benefit from future enhancements. The device also faced challenges related to signal instability due to waveguide interference, highlighting areas that require further refinement. Energy stability measurements underscored the importance of optimizing the laser power to maintain consistent signal quality. Despite adjustments, the device did experience some instability due to vibrations caused by a cooling fan of the excitation laser. These vibrations adversely affected the coupling efficiency, resulting in a noticeable loss of energy irrespective of the power setting. Attempts to detect the Doppler effect in moving microspheres were constrained by the sensitivity of the MRR and the size of the glass cover. These limitations hindered accurate detection of frequency shifts and time delays, indicating that future advancements in MRR design and manufacturing could significantly improve measurement accuracy. In conclusion, this research advances the field of photoacoustic imaging by optimizing system performance and identifying critical areas for future development. The improvements made and challenges encountered provide valuable insights for enhancing non-invasive vascular research and diagnostic imaging applications, setting the stage for more accurate and versatile PAM systems in the future.