High axial resolution swept source for optical coherence tomography

Abstract

A high resolution simple swept source for OCT measurements is presented. Low skew frequency domain triggering via electronic processing allowed resolutions of one FFT bin for a single reflection. The high resolution capability was demonstrated in a system containing 10 mm spaced interfaces.Introduction: Optical coherence tomography (OCT) has become an important characterisation tool for transparent micro-structured materials and biological applications [1]. In an analogy with ultrasound technology, optical reflectivity data provides information on the axial position of reflections within the sample (A-scans), whereas lateral position control gives rise to two-dimensional (B-scans) or three-dimensional (C-scans) images of the sample. Swept source OCT (SSOCT) [2] is based on longitudinal reflectivity data obtained with optical frequency domain reflectometry (OFDR) [3,4]. In this technique, the optical frequency of the source is swept and passed into two arms, sample and reference, of a Michelson interferometer. A beat frequency is generated when the reflections from the sample and the reference are recombined and detected. The frequency of the beat signal gives the relative delay of the sample and reference reflection. In the simplest and fastest setups, data is acquired at equal time intervals and an FFT operation gives the beat signal spectrum. The axial resolution of the technique is then strongly limited by the linearity of the optical frequency sweep [5]. To overcome this resolution limitation a time to frequency conversion operation is needed. Towards this end, part of the optical signal can be routed to an external etalon, the free spectral range oscillations of which can either be sampled along with the data and used to post-process an optical frequency scale [6], or the zero crossings of the oscillations can be used directly as sampling triggers for the data acquisition, allowing the data to be taken directly in optical frequency domain [3,7]. Certain laser designs are so repeatable in their sweep that a single calibration is all that is required to map specific time samples to frequencies [8] and others control their frequencies, making it possible to slave the laser to the data acquisition instead of the other way around [9].We report a new method to generate the optical frequency scale. In this case the constant frequency triggers are generated by the swept laser itself. The laser is calibrated once. From then on, regardless of sweep speed variations, the optical frequency triggers can be used to clock the A/D in the same way as an external etalon but without the need for extra optical components, a detector and zero crossing detection hardware.Experimental details: We are using a customised version of a New Focus 8700 external cavity diode laser with firmware modified to allow selection of a start and stop frequency and generation of constant frequency spaced triggers (10 GHz). The cavity is a modified LitmannMetalf design which rotates the end reflector about a pivot point with respect to a diffractio...

True logarithmic amplification of frequency clock in SS-OCT for calibration

“…Commonly the interferogram is sampled with equal temporal intervals by an analog-to-digital (A/D) converter, which means the data set is evenly distributed in time domain but not necessarily in the frequency or wavenumber domain. Unless the frequency/wavenumber is tuned linearly with time [ 13 , 14 ], direct FFT on this data set which is non-equidistantly distributed in the f or k domain, will introduce significant transform errors. These errors may dramatically compromise the performance of a SS-OCT such as its axial resolution and signal to noise ratio ( SNR ) [ 9 , 15 – 17 ].…”

Optical Sources for High-Resolution Optical Coherence Tomography

“…Position of Figure 5.17 A-scan is also indicated. [Cordes, A.H. et al (2010) OCT uses partial reflections of light from interfaces between dissimilar materials inside a sample under test to determine not only some character of each interface but also of where in the sample the interface resides.…”

“…We created two very different sources for Swept Source OCT. One was a New Focus External Cavity Tunable Diode Laser modified to profide realtime optical frequency markers which were used to clock the A/D [Cordes, A.H. et al (2010)]. The other was a stretched pulse tunable laser formed by dispersively stretching the ultra-wideband output of a Santec Ultra-Wideband laser [Cordes, A.H. et al (2009)].…”