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.
Swept-source lasers are key components of the modern spectral-domain Optical Coherence Tomography (OCT) systems. Further progress of OCT technology requires swept-source lasers working at specific, application dependent, wavelength, which are also compact and low-cost. In this work, we report the development of a low cost, compact, semiconductor swept-source laser based on slots etched along the ridge. The laser consists of a central gain section, phase section, semiconductor-optical-amplifier (SOA) sections and mirror sections employing higher-order gratings. The design and optimization of different parameters of the laser structure such as ridge width/depth, slot width/depth, no. of slots and so on are performed before the fabrication process. The main aim in the design step was to ensure single mode operation while maximizing the tuning range. The above optimization is performed keeping in mind the narrow linewidth of the emission spectra and minimum laser length. Due to the nature of the surface gratings, this type of laser does not require any complex regrowth and expensive e-beam lithography steps during fabrication. The slot and ridge sections of the laser are etched in a single wafer process step and the laser is also monolithically integrable with other system components. Initial measurements have indicated the high quality of the slot geometry and stable single mode operation with high side mode suppression ratio (SMSR) of 25 dB. The full-scale development of this laser will allow increasing resolution and imaging depth of OCT systems.
We present here a combined theoretical and experimental study to investigate the influence of external optical feedback in a semiconductor swept-source laser. The applied feedback is shown to transfer the coherence between the subsequent modes and retain it along the full sweep. As a result, the technique can act as a solution to the de-coherence during the mode-hops observed in this kind of swept-source lasers thus noticeable increasing the image quality of Optical Coherence Tomography systems.
We show theoretically that optical feedback can be used to phase lock the successive modes of multi-section frequency-swept source lasers as a means to increase the coherence length. The time-gated feedback technique can be applied to transfer the coherence between the subsequent modes to retain the coherence along the full sweep or to synchronise two independent swept sources. In analogy with CW lasers, we derive an Adler equation describing the locking conditions. When the constant feedback is applied, the laser can operate in a self-mixing, mode-locking or chaotic regime, depending on the sweeping speed. In order to verify the theoretical results, we have developed an experimental set up and performed initial measurements with optical feedback.
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