The PS-SS-OCT system used for this work has already been described in our previous paper.2 Briefly, the light is polarized and then modulated continuously by an EOM (PC-B3–00-SFAP-SFA-130, EOSpace) operating at 6.7 MHz. The EOM can be driven from dc up to tens of Megahertz, and the limit is due to the required drive voltages which lead to large slew rates at a high frequency. More importantly, this device is a waveguide modulator rather than a crystal modulator, hence, it possesses a broadband frequency response even with low voltage drive waveforms of 20 V or less. In comparison, broadband crystal modulators require drive voltages of several hundred volts. Low-voltage crystal modulators exist, however, these are resonant devices which employ a high-Q resonant tank circuit to boost the low-voltage drive signal and thus, only work at a specific harmonic frequency. It is then extremely difficult to drive these synchronously with an external timing signal. The modulated light is split into the reference and sample arm, recombined, and detected. The theoretical description and data processing procedures of the system have been previously described.2,5 The depth-resolved Jones matrices algebraically calculated from the experimental data can be described as,2Display Formula
1where α is the wavenumber-sweeping rate of the light source (–8.1×109 m−1 s−1), ε(n) is the acquisition timing offset of the n’th A-scan, Jsample is a depth-dependent double-pass Jones matrix of the sample, Jin and Jout are Jones matrices representing birefringence of the system fiber-optic components, and Joffset2 is a unitary matrix induced by the phase offset between the polarization modulation and the A-scan trigger and is characterized as
2where δ(n) is the phase offset of the n’th A-scan caused by the asynchronous driving between the ADC sampling clock and the EOM. To compensate the fiber-induced birefringence in the sample arm fiber, the Jones matrix at the sample surface is used as a reference matrix to calculate the birefringence in the sample. The Jones matrix at the sample surface can be expressed as
3We can only measure and depth-resolved . To obtain the phase retardance η and fast-axis orientation θ of the sample, matrix diagonalization is applied to the following equation,
4where p1,p2 are two transmittances of the eigenvectors of the sample, and JU is a general unitary matrix, whose columns are the fast and slow eigenpolarizations of Jc, m. θ is extracted from these eigenpolarizations. The degree of the phase retardance can be extracted through the phase difference of the resulting diagonal elements.