Abstract
We present an optical coherence tomography (OCT) method that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source. The method, based on the master slave interferometry technique, implements a coherence gate principle by requiring that the optical path difference (OPD) between the arms of an imaging interferometer is the same with the OPD in an interrogating interferometer. In this way, a real-time en-face OCT image can originate from a depth in the sample placed in the imaging interferometer, selected by actuating on the OPD in the interrogating interferometer, while laterally scanning the incident beam over the sample. The generation of the en-face image resembles time domain OCT, with the difference that here the signal is processed based on spectral domain OCT. The optoelectronic processor operates down-conversion of the chirped radio frequency signal delivered by the photo-detector. The down-conversion factor is equal to the ratio of the maximum frequency of the photo-detected signal due to an OPD value matching the coherence length of the swept source, to the sweeping rate. This factor can exceed 106 for long coherence swept sources.
Highlights
Master Slave (MS) optical coherence tomography (OCT) method was introduced to address the problems raised by using a Fourier transformation in processing the signal proportional to the spectrum modulation at the interferometer output
A novel practical implementation of Master Slave interferometry is presented where Mask signals are generated in real time while sweeping the frequency of the swept source
The coherence gate so created leads to an operation similar to that reported in en-face time domain OCT, with the difference that here spectral domain principle is employed: (i) spectra are measured and mixed and (ii) signal is acquired by working away from optical path difference (OPD) = 0 in each interferometer
Summary
Master Slave (MS) OCT method was introduced to address the problems raised by using a Fourier transformation in processing the signal proportional to the spectrum modulation at the interferometer output. In the initial report [1] introducing the MS operation, an interrogating interferometer was presented, and a comparison operator of the electrical signals delivered by the spectrometers or photo-detectors at the outputs of the imaging and interrogating interferometers. The MS method was evolved to a more practical procedure where the interrogating interferometer was replaced by the same imaging interferometer employed sequentially in two steps, Master and Slave. At the Master stage, a mirror is employed as sample and channeled spectra acquired for several OPD values are stored as masks. At the Slave step, when the sample replaces the mirror, the masks are compared with the acquired channeled spectrum. The comparison operation was performed digitally, via crosscorrelations or matrix multiplications [2] and several advantages of the MS methods have been reported, such as: (i) direct production of en-face OCT images (with no need to acquire the whole volume followed by software cut) [3], (ii) elimination of the need of resampling, and (iii) tolerance to dispersion in the interferometer [4], that makes the MS ideally suited to be used in conjunction with coherence revival [5]
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