Compact Optical Delay Line for More Efficient OCT Imaging
Andrew Robertson, Senior Vice President – Systems Technology Groups presents the G&H small form factor (SFF) delay line. The delay line was specifically designed for the exacting requirements of optical coherence tomography (OCT). OCT is an imaging modality, mainly used in biomedical applications. By using OCT, it is possible to take real-time, high resolution 3D images in a non-invasive manner.
OCT is a technique that utilises optical interferometry in which one interferometer arm is the reference optical path and the other is the optical path to the sample being imaged. The technique relies on the precise length alignment between the paths of the two interfering signals.
Depending on application, the sample could be the retina, the wall of an artery, the esophagus, or if being used for non-biomedical applications, the sample could be the coating of a pill, the surface of a photo-voltaic cell, or the surface of a flat screen display. In many of these applications, there will be changes in the path length of the sample arm during normal operation. As an example, when imaging the retina, there will be an unavoidable change in sample arm path length between subjects simply because of the difference in the size of a person’s head (e.g. child & adult) resulting in a difference in path length to the retina. Another, often unavoidable change occurs in OCT systems that use fiber optic catheters which may be several metres long – this is due to the manufacturing tolerance on the catheter lengths.
To ensure precise length alignment between the two interfering arms, one of the path lengths requires a variable optical delay line with high accuracy and high repeatability. At the same time, the unit has to be small enough and robust enough to be incorporated into equipment that will be used every day in demanding biomedical or industrial environments. To meet this need, G&H have developed the SFF optical delay line.
The SFF delay line has been developed for volume production line requirements. The unit is “plug- and-play” and can be controlled with a wide range of drivers through a standard connector. The unit is compact and features a built in attenuator, saving the cost and simplifying the design of the OCT system. The unit has been designed to minimize changes in transmission loss over both travel and temperature. The standard product can be configured in either a fiber to fiber configuration or point and return, depending on OCT system type.
All major OCT wavelength ranges are covered, including 850 nm, 1060 nm, 1310 nm and G&H can use all available fiber types and connectors for optimum integration into any OCT system. The SFF delay line travel speed is specified up to 25 mm/s with an accuracy of 5 μm and has been run up to speeds approaching 80 mm/s in short bursts. The standard variant offers optical delay of up to 1200 ps in point-and-return configuration.
The SFF delay line is one of a growing portfolio of components and sub-systems developed specifically for the OCT market by G&H. Other products include fiber splitters and tap couplers, fiber collimators, fiber interferometers, OCT spectrometers as well as custom opto-mechanical and electro-optical assemblies.