OCT measurements have a fundamental trade-off between the ability to resolve small details and the range over which the measurement is consistent. A measurement which is able to resolve small details is able to do so over a small range. A measurement which is consistent over a larger range is not able to resolve small details. While the axial resolution of the OCT measurement is determined by the optical bandwidth of the source, the lateral resolution (spot size) is determined by the focusing optics and the characteristics of the Gaussian beam. The spot size and the depth of field of a Gaussian beam are directly related in such a way that there is a trade-off between the spot size (details which can be resolved) and depth of field (distance over which the spot size is maintained). In this paper we analyze and discuss in detail the trade-off between the spot size and depth of field in OCT measurements. Some techniques to mitigate this limitation are mentioned and one is applied to measurements of cervical tissue with and without cervical intraepithelial neoplasia (CIN).

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...

The generation and detection of EHF microwave signals through the photomixing of two independent CW laser sources at commercial gallium-arsenide antennas is presented as a versatile technique for short range EHF communications. More than to eliminate further amplification before the irradiating antenna, the proposed technique is able to generate MMW signals at the EHF range with a maximum strength difference of 10 dB through the full range.

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The change in phase of the free space terahertz (THz) electric field as a sample of material introduced into the THz beampath of a CW THz system is measured and used to calculate the index of refraction of materials at 250 GHz.