| The high-speed modulation characteristics of semiconductor laser diode are studied and techniques to enhance the maximum speed are investigated. The three limiting factors for efficient high-speed modulation of semiconductor laser diodes examined here are parasitics, the matching circuit, and the photon relaxation oscillation frequency.; Parasitics are reduced by employing microwave packaging techniques which reduce the bond inductance and prevents additional package capacitance. A "Through-Load-Reflect" (TRL) calibration technique is implemented and thereafter, the laser chip Scattering-parameters are de-embedded. The parasitics effects on the modulation characteristics of the semiconductor laser diode are determined from the de-embedded Scattering-parameters of the laser diode chip.; A lowloss microstrip matching circuit with 90% bandwidth, for connecting to a laser diode of nominal impedance of 2 ohms to a 50 ohm system, is developed. The technique utilizes a microstrip Chebychev transformer without very wide line widths to obtain the match at a center frequency of 10.5 GHz with bandwidth of 9 GHz, insertion loss of less than 1.5 dB, and reflection coefficient of better than {dollar}-{dollar}10 dB.; A rate equation theory is developed for a semiconductor laser diode coupled to an external cavity under strong optical feedback. The theoretical and experimental results show that a semiconductor laser diode may be modulated at frequencies larger than its relaxation oscillation frequency. The laser diode under investigation had AR (anti-reflection) coating on one facet and a relaxation oscillation frequency of about 5 GHz, was placed in external cavities with fundamental resonances at 5 and 10 GHz. The external cavity enhances the rf modulation response at the frequencies corresponding to the external cavity resonance. The level of signal enhancement at the fundamental frequency is 20 to 25 dB, and at the second harmonic frequency is 30 to 38 dB. Large signal results show that the level of signal enhancement at the fundamental frequency remains high even at a modulation depth in the range of 10% to 20%. Furthermore, large signal results indicate, that it is not necessary to drive a laser at very large modulation depths to achieve significant harmonic enhancement. The region of modulation enhancement, with the external cavity, is about 10% bandwidth. Theoretically, a 24 GHz laser modulation is investigated, where the output amplitude is even higher than the low frequency level. The external cavity laser is shown to be a viable option for laser light modulation at millimeterwave frequencies. |
