Abstract：The technology of spectrum detection with high sensitivity is of significance in clinic diagnosis and tissue optical parameter measurement. A new method of difference-modulated laser spectrum detection was developed in the present paper. The measuring light and the reference light are separated from the lasing light source in this method. After passing through the tissue, the measuring light interferes with the reference light, and the frequency character of spectrum includes the information of the difference of optical path-length between the measuring light and reference light. By using the phase sensitive detector, the spectrum signal with different frequency can be separated, and consequently the measuring light passed through the tissue with different optical length will be apart. The mechanism of difference-modulated laser spectrum was analyzed and the value of dominant frequency of spectrum was deduced. Based on the theory of the optical path distribution in biological tissue, the spectrum signature of measuring light was discussed also. The distribution of dominant frequency component is decided by the difference of optical path-length between measuring light and reference light when the modulation parameters are invariable, and the magnitude of tissue’s modulus decay will effect the energy distribution of spectrum frequencies component. Theoretical analysis showed that the method of difference modulation can be used to separate lights according to the optical path-length and realize the single optical path measurement in biological tissue.
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