A DIY optical coherence tomography setup or system can be fun to build, but requires multiple optical and mechanical components, an understanding of signal and image processing, some programming expertise, and a significant investment of time. Using a prebuilt, off-the-shelf SD-OCT spectrometer as one of the starting components can speed and simplify this process, reduce risk, and improve the quality of images collected. In this tech note, we’ll introduce you to some of the key principles and layout of a DIY optical coherence tomography setup and share our expert tips on how to get the best performance from the OCT spectrometer portion of your design – whether you plan to build the spectrometer yourself or buy one.
Optical coherence tomography is an optical version of ultrasound, replacing the sound waves with light. Although OCT can’t probe as deeply (typically millimeters), it delivers images with much higher resolution than ultrasound, without the need for direct contact or use of a coupling medium. It provides surface profiles and information about subsurface structure and uniformity that can be useful in medical applications like ophthalmology, cardiology, and research, as well as in industrial inspection.
OCT uses low-coherence interferometry with visible or infrared light in combination with scanning across the sample to generate a series of cross-sectional images or 3D volume. There are several ways low-coherence interferometry can be performed, but two of the most popular ways are swept-source (SS-OCT) and spectral-domain (SD-OCT). Both methods shine laser light of multiple wavelengths on the sample and then measure the different wavelengths of scattered light that are returned, taking a Fourier transform of the spectrum to detect structures at different depths.
In swept-source OCT (SS-OCT), the laser is scanned in wavelength, and a single-element photodetector captures the signal. In spectral-domain OCT (SD-OCT), a broadband laser source is used in combination with a multi-element detector like a spectrometer. SS-OCT offers high speed and low roll-off (we’ll explain that term later) but is expensive due to the high cost of swept-source lasers. SD-OCT yields better resolution at lower cost, but it wasn’t able to compete on speed and roll-off performance – until better imaging cameras were introduced, several years ago. At Wasatch Photonics, we were the first to integrate these new cameras into our Cobra-S OCT spectrometer product line to offer industry-leading SD-OCT, starting in 2017.
In an SD-OCT system, broadband laser light is split between two paths: one to a reference arm and one to the sample to be measured. Light returning from these two paths is recombined and interferes to generate a fringe pattern which is read by the spectrometer, which digitizes the intensity at each wavelength. Either a supercontinuum laser or lower cost superluminescent diode (SLED) can be used as the light source. To generate a cross sectional OCT image, the laser beam must be scanned quickly across the sample with a high-speed scanning mechanism such as a galvanometer or MEMS-based device.
Software must be written to communicate with both the spectrometer and scanning arm to synchronize data collection with the beam scanning, and to process the spectra collected by converting the frequency data to time data using fast Fourier transforms.
The key modules of the DIY SD-OCT system like SLEDs, optical couplers, and reference arms are produced with advanced manufacturing techniques and can be purchased off-the-shelf. The OCT spectrometer is a unique module that utilizes cameras and sensors produced via semiconductor manufacturing methods, along with custom optical elements in precise alignment. This makes the OCT spectrometer a particularly challenging part of the DIY OCT system, and worthy of discussion in more detail.
The OCT spectrometer used in a DIY optical coherence tomography setup has a huge impact on the quality of images collected – influencing the imaging depth, resolution, speed of image acquisition, and contrast in the image. Low ‘roll-off’ (or loss of sensitivity with depth) is the key performance parameter for any OCT spectrometer and is determined by the spectrometer design.
If you are designing an OCT system for commercial use, as a standalone ophthalmology or nondestructive testing instrument, or as complementary imaging for surgical guidance, you may have additional size and integration restrictions. That’s why we’ve designed a smaller, lighter, and highly robust OCT spectrometer specifically for OEMs. It offers comparable performance to our signature Cobra-S OCT spectrometer, and is offered in multiple models matched to available SLD sources for cost-effective OCT imaging in volume. Wasatch Photonics’ OCT spectrometers meet the stringent requirements for thermal and mechanical performance required in medical devices, and have been designed into devices that are compliant with regulations like IEC 60601. Explore our ‘compact without compromise’ OEM OCT spectrometers.
Whether you have a new application or an existing system, our experienced team will help you find the ideal OCT grating or spectrometer for your research or new product development. Contact us today to discuss your unique needs.
Dr. Nishant Mohan is an accomplished optical scientist and biomedical engineer with over 15 years of experience in OCT imaging. He has authored numerous peer-reviewed publications and a patent in OCT techniques, demonstrating his commitment to innovation and scientific rigor in the development of cutting-edge photonics solutions.
