Swept Source OCT System Chapter 2: System Description
Rev H, June 3, 2011 Page 9
2.2. Swept Source Optical Coherence Tomography (SSOCT) Theory
Swept Source Optical Coherence Tomography (SSOCT) technology uses a rapidly tuned narrowband source to
illuminate the interferometer and records the information with a single photodetector. SSOCT technology, like
optical frequency domain reflectometry, 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 SSOCT system to enable
real-time video rate OCT imaging. This OCT system 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.
SSOCT has the advantage of generating high-speed depth profiles at the sweeping rate of the laser, as well as
collecting interference signals from the sample using a high-efficiency balanced detection scheme. The 5 –6 mm
coherence length of the laser enables approximate 3 mm depth measurement range of the reflected signal
without significant decay in the system detection sensitivity.
The Swept Source Optical Coherence Tomography system utilizes the latest swept source based Fourier domain
OCT technology to provide an OCT imaging system with detection sensitivity and imaging speeds much higher
than conventional time-domain OCT (TD-OCT) systems. At the heart of the instrument is a swept laser source
that tunes the lasing wavelength across a broad wavelength range at tens of kilohertz repetition rate. Each sweep
of the laser wavelength provides a depth scan at a sample surface point that yields a detailed depth dependent
reflectivity profile along the direction of the laser illumination path. The high-speed scan of the laser enables the
real-time video rate imaging speed, which is one of the most important features of the SSOCT system.
The SSOCT 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 OCS1300SS provides 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 CMOS
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.
2.3. Operating Principles
Figure 2, found on page 10, shows the schematic of Thorlabs’ OCS1300SS OCT system. This system
incorporates a high-speed frequency swept external cavity laser which has a 3 dB spectral bandwidth - (larger
than 100 nm) - and an average output power of 10 mW. The swept source has a built-in Mach-Zehnder
Interferometer (MZI, Thorlabs INT-MZI-1300) that provides the frequency clock for the laser. The main output of
the laser is coupled into a fiber-based Michelson interferometer and split into the reference and sample arms
using a broadband 50/50 coupler (Thorlabs INT-MSI-1300).
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 handheld probe, collimated, and then directed by the XY galvo
scanning mirrors towards the sample. The axial scans (A-scans) are performed at 16 kHz, which is the sweeping
frequency of the laser. The transverse scan (B-scan) is controlled by the galvo scanning mirrors and determines
the frame rate of the OCT imaging.
The light is then focused onto the sample surface by an objective with a long working distance. The long working
distance of the objective provides a large clearance (≥25 mm) between the optics and the sample, which enables
easy handling of 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. An aiming laser
centered at 660 nm is coupled into the sample arm of the interferometer to visually indicate the scanning trace of
laser.
