Jacob Stefan, Johansson Cecilia, Ulfendahl Mats, Fridberger Anders
Karolinska Institutet, Center for Hearing and Communication Research, Department of Clinical Neuroscience and Department of Otolaryngology, M1 Karolinska University Hospital, SE-17176 Stockholm, Sweden.
J Neurosci Methods. 2009 May 15;179(2):271-7. doi: 10.1016/j.jneumeth.2009.02.002. Epub 2009 Feb 13.
Laser interferometry is the technique of choice for studying the smallest displacements of the hearing organ. For low intensity sound stimulation, these displacements may be below 1 nm. This cannot be reliably measured with other presently available techniques in an intact organ of Corti. In a heterodyne interferometer, light is projected against an object of study and motion of the target along the optical axis causes phase and frequency modulations of the back-reflected light. To recover object motion, the reflected light is made to interfere with a reference beam of artificially altered frequency, producing a beating signal. In conventional interferometers, this carrier signal is demodulated with analog electronics. In this paper, we describe a digital implementation of the technique, using direct carrier sampling. In order to obtain the necessary reference signal for demodulation we introduce an additional third light path. Together, this results in lower noise and reduces the cost of the system. Within the hearing organ, different structures may move in different directions. It is therefore necessary to precisely measure the angle of incidence of the laser light, and to precisely localize the anatomical structure where the measurement is performed. Therefore, the interferometer is integrated with a laser scanning confocal microscope that permits us to map crucial morphometric parameters in each experiment. We provide key construction parameters and a detailed performance characterization. We also show that the system accurately measures the diminutive vibrations present in the apical turn of the cochlea during low-level sound stimulation.
激光干涉测量法是研究听觉器官最小位移的首选技术。对于低强度声音刺激，这些位移可能低于1纳米。在完整的柯蒂氏器中，目前可用的其他技术无法可靠地测量这一位移。在外差干涉仪中，光投射到研究对象上，目标沿光轴的运动会导致背向反射光的相位和频率调制。为了恢复物体运动，使反射光与人工改变频率的参考光束干涉，产生拍频信号。在传统干涉仪中，该载波信号用模拟电子器件解调。在本文中，我们描述了该技术的数字实现方式，即使用直接载波采样。为了获得解调所需的参考信号，我们引入了一条额外的第三光路。这共同降低了噪声并降低了系统成本。在听觉器官内，不同结构可能向不同方向移动。因此，有必要精确测量激光的入射角，并精确确定进行测量的解剖结构的位置。因此，干涉仪与激光扫描共聚焦显微镜集成在一起，使我们能够在每个实验中绘制关键的形态测量参数。我们提供了关键的结构参数和详细的性能表征。我们还表明，该系统能够准确测量低水平声音刺激期间耳蜗顶部微小的振动。
