Kirkby D R, Delpy D T
Department of Medical Physics and Bioengineering, University College London, UK.
Phys Med Biol. 1996 May;41(5):939-49. doi: 10.1088/0031-9155/41/5/009.
This paper describes an opto-electronic cross-correlator designed for measurement of the temporal point spread function (TPSF) of light at the bedside. Ultra-short (< 2 ps) pulses of light from a mode-locked laser were used to illuminate a tissue phantom. The light exiting from the tissue phantom was coupled by an optical fibre to a small-area (200 microns diameter, Hamamatsu S2381) avalanche photodiode (APD). The gain of the photodiode was modulated at the repetition rate of the pulsed laser (82 MHz). Usually the gain was approximately 100, but for a period of approximately 130 ps (FWHM) the gain was increased to approximately 105. A lock-in amplifier, which sampled the integrated APD current, gave an output proportional to the difference between the current in the low- and high-gain states. Hence a small section of the TPSF was selectively sampled. An overall temporal resolution of 275 ps FWHM was achieved. As the timing of the gain modulation was controlled by an all-electronic variable-time-delay system, the whole of the TPSF could be sampled without requiring any moving prism or mirror which is typical of many cross-correlators. Hence the system is mechanically very rugged, which enhances its durability in a portable instrument.
本文描述了一种用于在床边测量光的时间点扩展函数（TPSF）的光电互相关器。锁模激光器发出的超短（<2 ps）光脉冲用于照射组织模型。从组织模型出射的光通过光纤耦合到一个小面积（直径200微米，滨松S2381）雪崩光电二极管（APD）。光电二极管的增益以脉冲激光的重复频率（82 MHz）进行调制。通常增益约为100，但在大约130 ps（半高宽）的时间段内，增益增加到约105。一个对APD积分电流进行采样的锁相放大器给出的输出与低增益和高增益状态下电流的差值成正比。因此，TPSF的一小部分被选择性地采样。实现了275 ps半高宽的整体时间分辨率。由于增益调制的定时由全电子可变时延系统控制，所以无需像许多互相关器那样使用任何移动棱镜或镜子就能对整个TPSF进行采样。因此，该系统机械结构非常坚固，这提高了其在便携式仪器中的耐用性。
