Researchers have shown that a vertical cavity surface emitting laser (VCSEL) diode could reduce the cost of eye imaging systems by tens of thousands of dollars, allowing them to be used at home.
The team explained in Optics Letters that a VCSEL diode, commonly used in telecommunications and lidar applications, could be used for swept-source optical coherence tomography (OCT) – an imaging method that involves using a laser that changes wavelengths over time to visualise the eye.
Although swept-source OCT can help to diagnose eye disorders, the lasers required often cost tens of thousands of dollars. Now, however, the researchers say VCSEL diodes could reduce this price to just a few dollars.
“OCT has become a standard imaging tool in ophthalmology for diagnosing and monitoring diseases like glaucoma, macular degeneration and diabetes-related retinopathy,” said research team member Milana Kendrisic from the Medical University of Vienna. “However, the price of laser sources and the complexity of OCT systems hinder its accessibility outside of clinics and hospitals, in particular for home care or point-of-care use.”
Kendrisic continued: “We want to develop a low-cost OCT device that could be used in a general practitioner’s office, a pharmacy, or even a supermarket to allow eye checkups without going to the ophthalmologist. It could also allow home-based treatment monitoring for patients with age-related macular degeneration or eye health in diabetes patients to monitor treatment or catch abnormal changes early, avoiding loss of vision and ultimately preserving quality of life. This might be done by renting an easy-to-use device from your insurance provider, for example.”
Researchers have shown that a vertical cavity surface emitting laser diode could offer a less expensive light source for swept-source optical coherence tomography. Advanced post-processing algorithms (c, d) were used to improve the resulting images. (Image: Milana Kendrisic, Medical University of Vienna in Austria)
Single-mode VSCELs usually operate using a constant current, but the researchers varied the current to achieve the changes in the wavelength, or colour needed for swept source OCT. Other groups have demonstrated that VCSELs can be used for OCT, but this is the first time they have been used for eye imaging.
While VCSEL diodes typically produce a laser output beam at a certain well-defined wavelength, if the diode’s operating temperature changes, the output wavelength will also change.
Kendrisic explained: “Our idea was to leverage an unwanted effect of wavelength shifting with temperature to make these diodes usable for OCT. “We deliberately induce a temperature change by ramping up the driving current in a short time to high values beyond its original specifications. This is followed by a cooling period to avoid deterioration of the diode material, which lets us tune the diode over a broader bandwidth than is otherwise possible.”
The eye is mostly filled with water, so the researchers used a VCSEL that operates at 850nm – a wavelength that isn’t absorbed strongly by water. This allowed the laser light to travel through the eye to the retina and back to the detector without losing the signal to absorption.
“Even though commercial swept sources have larger spectral bandwidths, we showed that even the small range we achieved with a VCSEL is sufficient to perform OCT,” said Kendrisic. “These laser diodes could cost as little as a few dollars each if bought in bulk and also allow large distance ranging, which is needed for covering the full length of the human eye from cornea to retina.”
To demonstrate the VCSEL diode, the researchers integrated it into an optical setup for ophthalmic OCT imaging, which they say performed well in terms of sensitivity and imaging depth. They then tested the system on a healthy volunteer and obtained biometric information, including axial eye length and anterior chamber depth, with precision comparable to commercial OCT systems. According to the team, the results indicate potential applications in biometric eye scanner systems for security purposes.
“Although VSCEL-based systems have some limitations in terms of image quality, they still have the potential to expand access to diagnostic imaging and improve patient care in areas where traditional OCT systems may not be available or affordable,” said Kendrisic. “Our paper emphasises the use of this technology for ophthalmic applications, but lowering the price of OCT devices will allow an easier introduction of OCT technology in other fields as well – both medical and technical.”
Before a VSCEL-based system can be commercialised for eye exams, large-scale clinical studies and source lifetime tests will need to be performed to prove the system’s clinical relevance and longevity, the researchers say.
