Widely tunable single mode emission despite the small free spectral range of a long multisection slotted laser operating near 850 nm

Optical Coherence Tomography (OCT) is a technique used to create 2D and 3D images of biological samples. OCT can be performed using multi-section swept source lasers. We discus the challenges associated with such devices that operate near 850nm, which are of interest in ophthalmology as they have a low absorption rate in water and the short wavelength leads to good spatial resolution. In a typical multi-section swept source laser there is a Gain Section bounded by two Mirror Sections. The Mirror Sections have slots etched into them and act like Bragg Reflectors. The reflected wavelengths can be tuned by changing the refractive index of each Mirror Section, achieved by changing the temperature through the applied current. To ensure single mode operation, longitudinal modes must be sufficiently spaced so that only one mode lies beneath the overlapping mirror reflection peaks. A laser operating near 850 nm must be approximately a third of the size of a similar 1550 nm laser in order to achieve similar mode spacing. A shorter device has less gain available to compensate for lossy slots and since the Mirror Sections will be closer to each other there can be an increase in thermal cross talk. Stronger slot reflectivity reduces losses and reduces the width of mirror reflectivity peaks which defines the minimum cavity length required for single mode lasing. Different cavity lengths are investigated and the widths of the reflectively peaks are found.