Modelling optical coherence tomography for biophotonics and photobiology using an electronically tunable mode-locked laser diode

We propose mode-locked laser diodes (MLLDs) for their deployment in a low-cost and portable optical coherence tomography (OCT) system. OCT is an essential imaging technique used for medical diagnoses in dermatology, ophthalmology, and cardiology. Based on low-coherence interferometry, OCT directs infrared light through various layers of tissue, which is reflected onto a detector and resolved as an image. Generally, swept-source OCT (SS-OCT) systems perform better than grating based systems and time domain OCT but require expensive laser sources which are optically pumped, meaning they require an additional pump laser, limiting their deployment in clinics. To that end we propose MLLDs as an excellent candidate to realize, low-cost, compact, and portable SS-OCT enabled by their fast electronic tuning and electrically pumped, monolithic construction. We present simulated SS-OCT images using experimentally measured spectra from our InAs Quantum-Dot MLLDs and compare this to simulated data using a Thorlabs research-grade micromechanically tuned VCSEL (vertical cavity surface emitting lasers). Our first results to date suggest MLLDs could resolve features of 62.5 µm, which, compared with the off-the-shelf system, is approximately half the resolution. Further studies suggest that by examining electronic fine-tuning of the spectral linewidths and central wavelength, MLLDs may be highlighted as a key tool in realizing low-cost portable OCT at comparable quality to existing research-grade systems. Couple this with the current shift in practices to complex image analysis using machine learning methods, a handheld SS-OCT system could be realized as a low-cost, compact and versatile tool for clinicians.