SS-OCT System Base Unit
Chapter 3: Description
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3.1.2.
Thorlabs SS-OCT System Technology
Swept Source Optical Coherence Tomography (SS-OCT) technology uses a rapidly tuned broadband source
to illuminate the interferometer and records the information with a balanced detector. SS-OCT technology
measures the magnitude and time delay of reflected light in order to construct depth profiles (A-scans) of the
sample being imaged. Adjacent A-scans are then synthesized to create an image.
Advanced data acquisition and digital signal processing techniques are employed in the SS-OCT system to
enable real-time video rate OCT imaging. The 2D OCT images are analogous to ultrasound images and show
the cross-sectional structure view of the sample. The transverse and axial resolutions of the images are limited
by the focusing optics and spectral bandwidth of the light source respectively. The actual imaging depth into the
sample is highly dependent on the sample scattering properties at the measurement wavelength. The OCT
system also enables the generation of images similar to confocal microscopy by summing signals in the axial
direction. High-speed 3D OCT imaging provides comprehensive data that combines the advantages of surface
microscopy and structural OCT imaging in a single system.
At the heart of the SS-OCT system is a swept laser source that tunes the lasing wavelength over a broad
wavelength range, at hundreds of kilohertz repetition rate. Sample depth profile measurements are performed
at the high sweeping rate of the laser. The interference signals from the sample are collected using a high-
efficiency balanced detection scheme. Each sweep of the laser wavelength provides a depth scan at a sample
surface point that yields a depth dependent reflectivity profile along the direction of the laser illumination path.
The SS-OCT system utilizes the latest MEMS-VCSEL swept laser as the light source to perform Fourier domain
OCT measurements. The very short cavity length (on the level of a few micrometers) of the MEMS-VCSEL
cavity enables very long coherence length (≥100 mm) of the source when sweeping at very high speed (a few
hundreds of kHz). The very long coherence length of the MEMS-VCSEL swept laser supports the large depth
measurement range in OCT experiments, as its distinctive advantage compared to conventional short external
cavity (on the level of a few millimeters) based swept sources. The MEMS-VCSEL SS-OCT system is capable
of providing highly detailed, 2D cross-sectional imaging of a sample’s internal structure, as well as computer
generated 3D reconstruction of a volume near the sample surface. The internal structure of a sample can be
accurately mapped via computer generated tomographic images.
The VEG200 Series Swept Source OCT systems (Vega) provide simultaneous multiple imaging channels for
microscopic viewing of the sample. The
en-face
images, similar to those obtained from a conventional
microscope, can be acquired from the video camera channel while the cross-sectional images that show the
sample's internal structure are acquired from the OCT channel. Due to the novel data acquisition and signal
processing methods employed, real-time video-rate imaging speed has been achieved on both channels.
The system includes a high-speed MEMS-VCSEL swept laser with -10 dB spectral bandwidth larger than 100
nm, and an average output power greater than 20 mW, sweeping at ≥100000 A-scans per second. The swept
laser provides an A-scan trigger signal to be connected to the trigger input of the data acquisition device. The
laser also has a built-in monitoring interferometer that provides the real-time clock signals to be connected to
the external clock input of the data acquisition device. The main output of the laser is coupled into a fiber-based
Mach-Zehnder interferometer located inside the imaging module. The light is split into the sample and reference
arms using a broadband coupler.
In the reference arm of the interferometer, the light is reflected back into the fiber by a stationary mirror. In the
sample arm, the light is fiber-coupled into the imaging scanner, collimated, and then directed by the XY
galvanometric scanning mirrors towards the sample. A dichroic mirror is inserted into the beam path to reflect
the visible light from the sample onto a CCD camera that records the conventional microscope images of the
sample. The axial scans (A-scans) in the depth direction are performed at the sweeping frequency of the laser.
The transverse scan (B-scan) is controlled by the galvanometric scanning mirrors and determines the frame
rate of the OCT system. The light that exits the imaging scanner is focused onto the sample surface by an
objective.
