The recent advancements of swept-source optical coherence tomography (SS-OCT) calls for a compact widely tunable swept-source that can emit single mode, and narrow linewidth radiation over a 70-100 nm wavelength tuning range. However, the gain bandwidth of the laser material critically limits the wider tuning range of the device. Moreover, to ensure the single-mode operation, the mirror sections of the laser should be designed with a specific free spectral range (FSR) which further limits achieving wider tuning range of the device. These limitations drive the laser manufacturer to opt for external filters which restricts the speed of operation of the device. In this work, we have discussed an alternative approach of increasing the tuning range of the device without adding further complexities in the laser design by employing the method of electro-optic synchronization. Two tunable semiconductor lasers with different epitaxial structures and central wavelengths have been electrically synchronized and their outputs have been coupled optically. The tuning range of these lasers partially overlaps with each other for a smooth transition of the laser emission. Two multi-section semiconductor lasers with a central wavelength of 830 nm and 862 nm respectively, and with a tuning range of 40 nm, have been fabricated using standard UV optical lithography to be utilized in this approach. Initial electro-optic characterization of the lasers shows single mode emission with high SMSR throughout the tuning range.
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.
Quantum dot lasers have proven to be extremely interesting in terms of nonlinear dynamics. One of the key benefits is their highly damped relaxation oscillations. In this work we take advantage of this stability in a master slave configuration. We find a novel, dual state excitable dynamic resulting in antiphase operation between two lasing states in the device. We interpret the behaviour as a non-Adler but still Type I excitability. Phase measurements bolster our interpretation.
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