Wavelength locking and determination in tunable lasers by gain voltage measurement
In this thesis, tunable lasers under optical feedback are investigated. The influence of feedback on the spectral behaviour of the voltage at the gain section, the output power, the threshold current, the linewidth and the side mode suppression ratio are investigated. The findings complement the existing theory under the aspect of continuous wavelength tuning and the influence of a reflection with a fixed distance to the laser cavity. Furthermore, a more comprehensive theory of the internal and mirror losses is developed to explain the variations in the voltage curves. The theory shows good agreement with the measurement results. The device under investigation is a hybrid indium phosphide/polymer distributed Bragg reflector laser, developed and fabricated at the Heinrich Hertz Institute. Based on the theory developed, a virtual prototype of the laser is created. The mathematical model allows the calculation of the wavelength, voltage, and output power mappings, which are in good agreement with the measurement results. Further investigations of the virtual prototype resulted in several design rules with which the laser remains continuously tunable even under optical feedback. The findings of this work on the voltage variations under optical feedback are applied in experiments where a proof of concept for wavelength stabilisation is carried out as well as the determination of the absolute laser wavelength. No additional, and in particular, no optical instruments were needed for the wavelength stabilisation and determination. Only the voltage at the gain section is used, which created the basis for self-wavelength stabilisation and determination.
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