Thorlabs offers a wide variety of Optical Coherence Tomography (OCT) imaging systems. We recognize each imaging application has their specific needs. With the growing number of OCT systems available, it can be challenging to decide which system best meets your needs. Below we have put together a Selection Guide that outlines a few key technical specifications of each of our systems as well as some tips on how to choose the best OCT system for your application.

Optical coherence tomography

OCT Cross-Sectional Image of a Human Finger. Layers of Skin: E-Epidermis; D-Dermis; BV-Blood Vessels. Image Size: 4.9 mm x 2.6 mm. Image Taken with a Telesto Series OCT System.

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The sensitivity of an OCT system describes the largest permissible signal attenuation within a sample that can still be distinguished from the noise. In practice, higher sensitivity OCT systems are capable of providing higher contrast images. Since the sensitivity of an OCT system can be increased by increasing the integration time, there is usually a tradeoff between A-scan rate and sensitivity.

The length (L) and width (W) of the FOV are limited by the scan lens properties. Most of our OCT systems have a 10 mm x 10 mm (L x W) FOV while the GANYMEDE-II-HR offers 6 mm x 6 mm (L x W) FOV in the default configuration. The maximum depth (D) attainable is set by the design of the OCT system. The graphic to the right shows variation in depth among all of our OCT systems. However, the actual imaging depth will typically depend on the optical properties of the sample. Our standard OCT systems are designed to provide an optimized balance between imaging depth and axial resolution. For applications requiring greater depth or higher resolution, we offer custom configurations.

Thorlabs currently offers OCT systems that operate with a center wavelength of either 900 nm, 930 nm or 1300 nm. The center wavelength contributes to the actual imaging depth and resolution of the system. Shorter wavelength OCT systems, such as our 930 nm or 900 nm systems, are ideal for higher resolution imaging compared to systems with a center wavelength of 1300 nm. For imaging samples that have higher optical scattering properties, such as tissue, the longer wavelength systems are recommended. The longer center wavelength is not affected by scattering, and therefore, the light is able to penetrate deeper into the sample and return for detection.

In dark-field microscopy, the inner part of the illuminating light beam is blocked by a mask. Thus, only a sheet of light is focused onto the sample. In other words, the sample is illuminated under an angle that is defined by the NA of the objective lens. Only the fraction of light is detected, which has changed its angle upon interaction with the sample. The image contrast is primarily based on light scattering, enhancing the contrast of, e.g., small particles and sharp edges.

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In OCT, the axial (depth) and lateral resolutions are dependent on different factors. The axial resolution of the OCT system is proportionally dependent on the center wavelength of the source and inversely proportional to the source bandwidth. In practice, the axial resolution is also improved by the index of refraction of the sample. For example, the axial resolution of the CALLISTO OCT system is 7 ?m in air or 5.3 ?m in water-rich samples such as tissue (n=1.35).

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OCTprinciple

A single depth profile (Intensity vs Depth) is called an A-Scan. A B-Scan, or two-dimensional cross-sectional image, is created by laterally scanning the OCT beam and collecting sequential A-scans. The speed with which a B-scan is collected depends on the A-Scan or Line rate.

As with general microscopy principles, the lateral resolution is dependent on the focusing objective in the imaging probe. All of Thorlabs’ OCT systems come with our specially designed OCT scan lens which provides telecentric scans across the entire field of view.

The spectral bandwidth of the OCT light source is indirectly proportional to the axial (depth) resolution of the imaging system. Therefore, broadband light sources are used to provide high axial resolution.

In normal bright-field microscopy, light is focused by an objective lens (characterized by its magnification and numerical aperture (NA), e.g. 50x/NA=0.75) onto the sample and the light either reflected (reflected light microscopy) or transmitted (transmitted light microscopy) by the sample is collected. The image contrast is primarily based on light absorption.

For Spectral-Domain OCT systems, the A-Scan rate is determined by the speed of the camera in the detection spectrometer. For Swept-Source OCT systems, the A-Scan rate is determined by the sweep speed of the swept laser source. There is a tradeoff between A-Scan rate and the sensitivity of an OCT system: higher A-Scan rate results in lower sensitivity.

Thorlabs offers a wide variety of OCT Imaging Systems. To assist in narrowing down which OCT system(s) is best suited for your application, we have provided the guide below. We always encourage all customers to contact us to discuss specific imaging requirements.