Integrated tunable quantum-dot laser for optical coherence tomography in the 1.7µm wavelength region

In this thesis the results are presented that were obtained in the development of a novel integrated semiconductor laser light source suitable for the three-dimensional (3D) imaging technique of Optical Coherence Tomography (OCT) in biological tissues. The laser development was driven by new possibilities and requirements in OCT imaging which could be seen to be realizable through use of the latest developments in optical integration technology on Indium Phoshide (InP) and capabilities of quantum dot (QD) based integrated optical amplifiers. It has led to a fully integrated continuously tunable laser source in the 1600nm to 1800nm wavelength region on a single InP chip. For a particular version of OCT that allows for 3D imaging within a fraction of second, there are two main requirements. The first is that the laser must be able to scan over a wavelength range of at least 100nm to obtain sufficient depth resolution. The second main requirement is that the laser must be able to scan within several tens of microseconds over the whole tuning range repeatedly to obtain the imaging speed. To enable an increase in the OCT imaging depth into the tissue the development was aimed at a laser source in the 1600nm to 1800nm wavelength region where Rayleigh scattering is reduced. The goal was to realize a laser design based on a ring laser cavity with intra cavity quantum dot amplifiers and tunable filters within the generic active-passive integration technology of COBRA. It is demonstrated that this integration technology designed for the use in the 1550nm wavelength region can also be used in the 1700nm region without a large penalty in performance. The quantum dot amplifiers used are specially designed to generate and amplify light over a wide bandwidth centred at 1700 nm. To tune the wavelength of the ring laser the tunable filters are included. These electro-optical tunable wavelength filters are based on arrayed waveguide gratings (AWGs) with electro-optically phase modulators in the arms of the AWGs. This allows the control of the filter tuning over a bandwidth of over 200 nm and provides the possibility of tuning with the same speed for any wavelength step within its range. The research consisted of three major parts. The first is the realization, characterization and modelling of the QD amplifiers realised on a fully active wafer. It was shown that the quantum dot amplifiers are suitable for the task and the dependency of the gain spectrum on current density has been modelled successfully. The shift of over 100 nm observed in the gain spectrum with changing current has been explained satisfactorily using a coupled rate equation system and has revealed the role of the level structure in the dots and the coupling of those levels to a common charge carrier reservoir. In the second part the focus was on the design, fabrication and characterization of the electro-optically tunable filters. A combination of two filters is chosen to fulfil the requirements for the laser. The first filter is a high resolution filter with a 0.5nm full-widthhalf-maximum of the passband and a 10nm free spectral range. The second filter, a low resolution filter, is used to suppress unwanted passbands of the high resolution filter and has a 29nm full-width-half-maximum of the passband and a 210nm free spectral range. The lay-out of the filters was optimised for fabrication, the performance of the filters was tested and the results compared well with the design values. A dedicated measurement setup and electronics to control the 40 electro-optic phase modulators in the filters and laser chips was realised to accommodate the many connections between the electronics and the InP chip. The automatic calibration systems, control principles for the filter tuning and related software were developed. Tuning over more than 100nm is demonstrated with an accuracy of 0.1nm wavelength accuracy (1% of the free spectral range) for the high resolution filter and 9nm wavelength accuracy (4% of the free spectral range) for the low resolution filter. The switching of the high resolution filter was demonstrated with a 10%-90% rise and fall time within 100ns. The third part is the design, modelling, fabrication and characterization of the complete tunable quantum dot laser with integration of active and passive devices on a single InP chip. To simulate the tuning capabilities of the laser, the rate equation model of the quantum dot amplifier was extended to a laser model. The whole laser system was realised on a 6 x 10 mm2 chip and tested. It is one of the most extensive integrated optical circuits realised in active/passive technology in Europe. The laser was demonstrated to be tunable over 60nm between 1685nm and 1745nm with an 0.2nm wavelength accuracy and an effective linewidth of 0.11nm. The output power of the laser was approximately 0.1mW. Switching between two wavelengths in this 60nm was possible with a 500ns 10%-90% rise and fall time. Finally the tunable laser has been used in a free space Michelson interferometer setup. This is the first step towards an OCT measurement. The measurement of the mirror position in one of the interferometer arm was demonstrated with a 500Hz scan rate and 4000 wavelength sampling points over the 60nm scan. The performance of the tunable laser presented in this thesis is close to the required performance to be able to use the laser in an OCT measurement system. The results did not raise any fundamental limitations in the laser performance which makes the laser suitable for further development to a market competitive product.