Impact of Waveguide Input Coupling on Vertical Cavity Surface Emitting Laser Diodes

This diploma thesis deals with physical effects in the light output of a vertical cavity surface emitting laser (VCSEL) diode caused by external optical feedback. Such feedback might be induced by reflections from the beginning of a optical waveguide due to a connected refractive index step. The thesis also investigates the influence on waveguide coupled power on opto-electronic waveguide boards due to external feedback caused variations in the laser output. The work was initiated by measurements on opto-electronic waveguide boards, carried out at the institute, showing strong waveguide output power oscillations over board temperature. These measurements are reported in Chapter 1. Chapter 3 shows that such oscillatory behavior cannot be attributed to the laser diode as an intrinsic property, but must be the result of an interaction with the environment. A literature search showed that external optical feedback can be the source of oscillations in the laser output, if the feedback is coherent to the laser internal light and the relative phase between the two varies. The theoretical results found in literature are summarized in Chapter 2. This Chapter also provides a short introduction into the theory of VCSELs. Chapter 3 investigates the laser diode in detail. Results of static as well as dynamic measurements are reported showing changes in VCSEL output spectrum with drive current and modulation frequency. A connection between spectral lines and transversal VCSEL internal cavity modes is established in this Chapter, showing that every spectral line corresponds to a different mode. Therefore changes in the spectrum are equivalent to variations in the transversal modal power distribution or equivalently in the transversal beam intensity distribution. It is also looked into the temperature behavior of the device, showing variations in the laser spectrum and total power with temperature, but no oscillatory behavior. All external optical feedback measurement results are reported in Chapter 4. It is first shown that incoherent feedback has a strong influence on the laser output spectrum and power, but without any oscillations when varying the feedback phase or strength. Therefore coherent feedback is investigated afterwards, showing oscillatory variations in the laser spectrum with feedback phase, but no change in total output power. That leads to the assumption that a modal filtering element is necessary to translate the variations in the VCSEL transversal intensity distribution into corresponding total power variations after the filter. It is shown that a fiber and therefore also the waveguide in the opto-electronic waveguide boards can act as such an element and explain the power oscillations of the waveguide output light reported in Chapter 1. Finally Chapter 5 concludes the results and casts a light onto questions that could not be answered and would need further investigation.