| Heterodyne laser interferometry has the characteristics of high detection sensitivity and measurement precision,strong anti-interference ability,and can realize remote detection.It is increasingly used in the field of biomedical engineering widely.Traditional detection methods for axial length and photoacoustic signal are based on ultrasound technology.Such a contact-based signal detection method not only increases the risk of cross-infection,but also is not conducive to the development of clinical practice.In addition,the accurate measurement of axial length has important reference value for the evaluation and prediction of the refractive state of human eyes and the calculation of the preoperative intraocular lens power.Photoacoustic technology is based on the difference of light absorption inside biological tissue,which can present tissue images with multiple contrasts and high depth-to-space ratios,and endogenous chromophores are taken as targets to achieve functional imaging of tissue.The measurement of ocular biological parameters and photoacoustic imaging using optical technology will improve the detection accuracy and clinical comfort,and this research has strong clinical application value.Therefore,this paper aims to use laser heterodyne interferometry to achieve noncontact axial length measurement and photoacoustic signal detection.The axial length measurement system is a double-beam heterodyne interference system based on the architecture of an improved Twyman-Green interferometer.The measurement mirror is driven by a motor to move so as to generate doppler frequency shift,and the measurement of axial length is realized by detecting the peak position of the interference signal envelope.Based on this,the Q-switched Nd: YAG pulse laser is selected as the excitation source in the photoacoustic signal excitation system,to excite the sample to generate photoacoustic signal.For the photoacoustic signal acquisition,the dual-frequency He-Ne laser is used as the detection light source,and the heterodyne interference detection system is constructed to generate the beat frequency signal that carries the characteristics of photoacoustic signal.Then the photoelectric conversion and amplification of the interference signal are realized through the developed photoelectric detection module.Finally,the signal is acquired and stored using the high-speed data acquisition card.And the laser pulse signal is used as the trigger signal of the acquisition card to ensure the timing synchronization of excitation and acquisition.In the experimental study of the axial length measurement system,the axial length value was obtained by the low coherence interference signal demodulation algorithm,and the measurement accuracy was improved by setting the system signal-to-noise ratio threshold.Statistical analysis of axial length measurements in the right eye(the range of equivalent spherical mirror is-8.50 from 0.50 D)of 33 subjects showed that the axial length measurement system had good repeatability(Co V = 0.0617%,ICC = 0.9999),and compared with IOL master,there was no significant difference in axial length measurement results between the two devices(P>0.05),and had high correlation(r=1.000,P<0.001)and good consistency(the 95% confidence interval:-0.045 mm-0.056 mm).In the experimental study of the photoacoustic signal detection system,the photoacoustic signal was demodulated by the continuous zero-crossing method and the I/Q quadrature method.Firstly,the performance of the system was verified by simulated vibration signal from the ultrasonic transducer.The experimental results showed that the photoacoustic signal detection system can restore the ultrasonic vibration signal well.Compared with the OLYMPUS water immersion probe,the relative measurement error of the vibration frequency was small(0.09%),and the absolute difference was 0.45 KHz.Then,carbon rod and in vitro biological tissue were used as samples,and short-pulse light excitation was used to induce photoacoustic signal.The heterodyne system and ultrasonic probe were used for signal detection.The comparative analysis results showed that the heterodyne system in this paper can realize the non-contact and high-precision detection of photoacoustic surface vibration waves within the detection bandwidth.Overall,the detection system based on optical heterodyne interferometry combined with digital demodulation technology can measure the axial length and detect photoacoustic signal of the tissue in the case of non-contact.The performance of the system in axial length measurements with different health levels and different ages could subsequently be assessed by incorporating more subjects.In addition,the system structure and demodulation algorithm can be optimized to improve the detection bandwidth and demodulation accuracy,and the amplitude measurement results can be calibrated,so that a more comprehensive and objective system performance test result will be obtained. |
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