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The simple FF-OCT system described in this section is based on the time-domain OCT operating with optical heterodyne detection. It is equipped with the essential FF-OCT

components and constructed on an optical breadboard. Fig. 3.1 (see below) illustrates a schematic diagram and a photo of the developed table-top FF-OCT system which consists of a bulk Michelson interferometer, an infrared LED source for system illumination, a motorised stage for axial translation of the sample with a speedυs, a window as the reference mirror, and a CMOS camera at the interferometer exit.

Fig. 3.1 (a) Schematic diagram and photo of the developed simple FF-OCT system with an infrared LED source. BPF, band-pass filter; (b) photo of the Table-top FF-OCT System.

The infrared LED source was charged in series with a 5Ωresistor in a simple electric

circuit powered by a 3 V voltage and a 0.15 A current, providing an irradiance on the sample of 0.3 mW cm−2typically. The optical spectrum of the infrared LED source has a central wavelengthλ0 = 880 nm and a spectral width∆λ = 110 nm. The best focused spot of the light from the infrared LED source has a square shape. However, the LED does not have a uniformly-illuminated square aperture, as there is a loop-shaped electrode on top of the semiconductor chip to link to the anode lead of the LED. The focused spot on a sample and the formed image on the image sensor exhibited weaker illumination strength at the location of the electrode.

The CMOS camera (FMVU-03MTM-CS, Point Grey) is a high-speed camera recording 16-bit monochromatic digital data (10-bit valid grey levels, i.e. a 60 dB dynamic range). It grabs a typical raster image containing a fixed number of rows and columns of picture elements or pixels. Each pixel holds a quantized value that represents a shade of gray (varying from black at the weakest intensity to white at the strongest) at any specific point. The maximum frame rate allowed by the CMOS camera is 120 fps for an image size of

376×240 pixels, of which the corresponding pixel size is equivalent to 12 µm×12 µm after 2×2 pixel binning.

The motorised translation stage is controlled electronically and has a 50 mm linear travel along a well-defined axis. It can be operated to achieve an incremental movement as small as 0.1 µm. It can provide a smooth and constant friction movement under a slow, steady speed of 1 µm s−1. Both the camera and the stage are connected via USB to the PC, in which a control module is used to handle the operations in an OCT measurement, and a process module is used to process the acquired OCT data.

a. Interferometry Mechanism

Considering the light propagation within the designed FF-OCT system, the low-coherence light from the infrared LED source is first split into a reference beam and a sample beam by a non-polarising 50/50 beam-splitter. Light backscattered by the sample is recombined with the light reflected by the reference mirror at the beam-splitter and finally captured with a CMOS camera. Interference occurs only if the OPD between the reference and sample arms is within the coherence length of the light source. An on-screen interference pattern (see Fig. 3.1 (a)) indicates the matching point for the two interferometer arms.

b. System Alignment

An accurate optical alignment was carried out to assemble these system elements, i.e. aligning the reference arm with the light source and the beam-splitter and then aligning the CMOS camera with the beam-splitter and the sample arm. A laser beam, e.g. from a laser diode was used to assist the alignment of the optical axis to be parallel to the horizontal plane. Meanwhile, the positions of both lenses, the light source and the camera were roughly adjusted according to the calculated distances, which were derived from the required magnification and imaging FOV of the sample.

Further precise adjustments were made by the use of both the infrared LED source and the camera. A distance calibrator (R1L3S2P, Thorlabs) with horizontally sputtered micrometre divisions on a glass substrate was frequently used as the sample to calibrate the FOV and assist the alignment process. With an aligned and fixed reference mirror, first the position of the calibrator was adjusted along the optical axis until the strongest interference pattern was formed in the camera screen; then, with the reference beam covered up, the camera and the lens were adjusted to have the desired FOV identified by a sharp image of the micrometre divisions. Finally, with the reference beam uncovered and the sample beam shielded, the

light source and the lens were adjusted so that the luminous pattern within the LED occupied the full screen of the camera and was displayed as a sharp image.

c. Depth-Scanning Preparation

After the alignment process, the sample under test can be attached to the sample arm, replacing the distance calibrator. The depth-scan parameters, including the movement speed of the sample and the frame rate of the camera, need to comply with the requirement of the sample measurement.

Following the optical heterodyne detection in time-domain OCT (see Section 2.4.2 above), the modulation of the OCT signal is carried out by mixing with a strong local oscillator wave, which is experimentally generated by moving the sample or reference arm along the longitudinal direction. To digitise the interferogram using a photodetector with a fixed sampling rate, the depth-scanning can be accomplished by translating either arm towards the sample/reference matching point under a fixed slow speedυs. An effective digitalisation in FF-OCT is guaranteed by grabbing images at a frame rate νcam larger than twice the heterodyne beat frequencyνbeat (see Equation 2.12 above) according to the Nyquist rate:

νcam≥2νbeat = 4υs

λ0

, (3.2)

in whichλ0is the central wavelength of the source spectrum. The sensitivity-enhancement of the OCT signal can be fulfilled by sampling more points in an oscillating beat cycle. Hence, a slower translation speed and/or a faster camera frame rate are desired to increase the sampling rate of the OCT signal. However, the increased measurement time and/or data storage should also be considered according to the specific requirements in the actual measurements.

For most measurements using the simple FF-OCT systems, the CMOS camera was operated to successively grab 16-bit 240 px×240 px mono images with a pre-set frame rate of 120 fps. The stage in the sample arm could be moved at a steady speed towards the beam-splitter, such as 1 µm s−1, 3 µm s−1, or 5 µm s−1. It involved a trade-off between high axial precision and less data storage. Normally an overall measurement time of less than 1 minute can be achieved for probing the depth of a sample of less than 100 µm.