OCT (Optical Coherence Tomography) is an imaging technique that uses light interference to capture images of the internal structure of a sample with high resolution and high speed. In recent years, it has been used mainly for fundus examination equipment in ophthalmology. In addition, the non-contact, non-invasive, real-time imaging capability makes it applicable in various fields, such as visualization of the internal vascular structure of the skin for cosmetics manufacturers, cell observation for the biotechnology market, and shape measurement of metal working parts. It can be customized for various wavelength ranges from visible light to near-infrared, depending on each application.
Compact Spectrometer for OCT
OCT stands for Optical Coherence Tomography, which is a 3D imaging technique that allows for non-contact and non-destructive high-resolution measurements of the internal structure of a sample using the interference of light. With a few micrometer image resolution, OCT can measure structures in both surface and depth up to several millimeters. OCT has been developed primarily in the field of ophthalmology. It is also used in various fields, including industrial inline inspections, such as examining blood vessels, skin and cellular samples.
SD-OCT (Spectral Domain OCT) is a technique that involves directing broadband light onto the target and obtaining depth information by Fourier transforming the wavelength information from the interference light reflected from the target.
Basic configuration (Time-domain)
OCT uses low-coherence light sources like Super Luminescent Diodes (SLD). The beam from the source is split by a beam splitter, with one part directed toward the target and the other toward a reference mirror. The light entering the target is reflected or back scattered due to differences in refractive indices and scattering bodies within the target. The light from the sample and the reference mirror are overlapped. Intensities from the sample and the reference mirror are overlapped. The beams from the sample and from the reference mirror are interfered only when the travel times of light returning from the sample and the reference mirror are approximately equal. SLD is a low-coherence light source with a broad spectrum, similar to an LED, and high brightness, similar to a laser diode (LD).
To measure the internal structure, it is necessary to move the reference mirror along the optical axis intentionally. By varying the travel time of light from the reference mirror while measuring the light intensity at each position, interference occurs at different positions of the moving reference mirror due to differences in travel times of light returning from the internal structure of the target. The obtained light intensity, plotted against the movement of the reference mirror along the x-axis, provides information about the internal structure of the sample. Additionally, by scanning in the xy-direction using devices like galvanometric mirrors, 2D or 3D images can be obtained.
Fourier-domain OCT
Fourier-domain OCT (FD-OCT) is a technique that further enhances the speed and sensitivity comparing to those of Time domain OCT. Instead of moving the reference mirror, it uses a spectrometer or a wavelength-swept light source respectively to measure the intensity of interference signals over wavelength range and obtain an interference spectrum. Since the frequencies of the obtained interference spectrum correspond to the positions in depth of various boundaries within the sample, similar measurements to Time-domain OCT can be achieved by Fourier transforming this data. This allows simultaneous measurement of all positions in depth within the sample without the need for mechanical moving parts, resulting in real-time-like high-speed and highly sensitive measurements without unnecessary noise.
FD-OCT can be divided into two main categories based on their configurations: Spectral-domain OCT (SD-OCT) and Swept-source OCT (SS-OCT). SD-OCT utilizes a broadband light source and a spectrometer as a detector to simultaneously acquire interference spectra. On the other hand, SS-OCT measures the intensity of light at each wavelength by sweeping the wavelength in the light source and then arranges them temporally to obtain interference spectra.
These two techniques differ in resolution, measurement depth, stability, speed and ease of component acquisition. They are chosen based on specific purposes and applications. Moreover, recent advancements have led to the development of multifunctional OCT, which uses these techniques to not only capture structural information of the target but also obtain material properties and other valuable information.
Selection of Wavelength
For SD-OCT, the spectrometer is one of the key components that needs to be customized according to the measurement sample. First, you need to choose the wavelength range such as visible range, 800 nm range, 1000 nm range, 1300 nm range and so on. The choice depends on factors like the absorption and scattering characteristics of the sample, as well as the availability of components. For example, in retinal examinations, wavelengths around 800 nm or 1000 nm are often preferred to minimize the influence of water absorption.
Next, consider the depth resolution \(\delta_z\) and the depth range \(Z_{max}\). These are primarily determined by the light source and the spectrometer. Giving the center wavelength of the light source as \(\lambda_c\), the full width at half maximum (FWHM) of the light source as \(\Delta\lambda_L\), the pixel resolution of the spectrometer as \(\delta\lambda_s\) (spectrometer wavelength range \(\Delta\lambda_s\) divided by the number of detector pixels \(p\)) and the refractive index of the sample as \(n\), they can be expressed as follows:
\(\delta_z=n/2In2\) * \(\lambda_c^2/\Delta\lambda_L\)
\(Z_{max}=1/4n\) * \(\lambda_c^2/\delta\lambda_s\)
The two diagrams below show the depth resolution and the depth range for each center wavelength. To achieve better depth resolution, a broader bandwidth light source is required. However, a spectrometer designed for such a light source typically has larger pixel resolution, resulting in narrower depth range. As a result, there is a trade-off relationship between depth resolution and depth range.
A spectrometer that covers the entire wavelength range of the light source is not sufficient. For example, when using a light source with a bandwidth of 800-900 nm, you would require a spectrometer that covers a similar bandwidth. Using a spectrometer that covers a wider range, such as 700-1000 nm with the same sensor, would limit the depth range from what should be approximately 3.7 mm to just 1.2 mm. To maximize performance, it is crucial to have a spectrometer customized to match the characteristics of the light source being used.
Sensitivity
The sensitivity of the interference signal decreases with depth, primarily due to worse reproducibility of the interference spectrum in deeper site. As the depth increases, the interference spectrum contains higher frequencies. However, because the camera's pixel size and the spectrometer's collection spot size have finite dimensions, the reproducibility worsens with increasing depth, resulting in reduced signal intensity. This relationship between sensitivity \(R(z)\) and depth \(z\) can be expressed as follows:
\(R(z)=\frac{\mathrm \sin^2 \begin{bmatrix} D(z)\end{bmatrix}}{\mathrm D(z)^2}・exp \begin{bmatrix} - \frac{w^2}{2In2}D(z)^2\end{bmatrix}\)
\(w\) represents the ratio of wavelength resolution \(\delta\lambda_R\) to pixel resolution \(\delta\lambda_s\). Attenuation of the sensitivity calculated by this equation, for two focused beam spot sizes (= wavelength resolution, x1 and x2 compared to pixel size) are shown in the figure below. It is evident that the impact of Performance of beam focusing on sensitivity is substantial. Spectrometers required for OCT differ from typical spectrometers and need to be designed so that its focused beam size on the detector is very small, for example smaller than the pixel size.
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