External cavity laser diodes (ECLDs) have attracted much attention because they have considerably narrow linewidth and tunable single frequency output, and have been widely used in many applications including coherent communications, interferometric sensors, and spectroscopy. In an usual ECLD, dielectric anti-reflection (AR) coating on the facet of a laser diode is frequently executed for strong external feedback. To obtain the wide continuous tuning range without mode hopping in ECLD, the pivot point of the external reflector should be selected carefully.
In this thesis, two simple, but reliable, methods for evaluating the linewidth enhancement factor, α, value without modifying the laser configuration of conventional external cavity were suggested and demonstrated using the near infrared (785 nm) single mode ECLD. The unique dynamical characteristics of ECLD such as frequency tuning, spectral linewidth, FM noise, and stability problem result from the existence of the linewidth enhancement factor α of the LD. Consequently, it is very important to know the precise value of α to understand dynamics of the ECLD systems. In the first method of current scanning, the frequency tuning curve of ECLD as a function of injection current was used to evaluate the α value. We introduced a simple model which yields a linear relationship between the injection current and the phase shift of an incident wave after one round trip in the external cavity. With this assumption, experimental results like a multi-stability property or unmeasured wavelength regions could have been explained and the internal parameters of the solitary LD, α and $r_{int}$ were determined. On the other hand, in the second method of reflectivity scanning, the external feedback intensity was scanned to control the effective reflectivity of ECLD. The change of the external feedback intensity was accomplished by adjusting the rotation angle of a half wave plate (HWP) which was inserted in the external cavity. From the frequency tuning curve of ECLD as a function of the external feedback intensity, α and $r_{int}$ of the solitary LD were determined again. These values had worse statistical errors than those of the current scanning method due to the inaccuracy of reading the polarization angle of the HWP, but reasonably matched well to the previous result of the current scanning method. The values of α and $r_{int}$ of the both methods agreed with previously reported values.
The injection locking method and the mutual coupling method have been actively studied in many laboratories to satisfy the need of large output power and multiple beams of same wavelength in the application field of LD and to reveal the mutual-effect of coupled LDs. Until now, only weak and symmetrical coupling systems have been tried in experiment to avoid chaotic output characteristics which cannot be controlled.
Several types of strongly coupled system have been tested to obtain the stable and synchronized output beam. In the directly coupled case, two wavelengths of LDs were gone to close to each other but failed to reach to the perfect synchronization. A new structure consisted of two mutually coupled LDs in the cross shape was proposed and shown to make the perfectly synchronized output. Since this structure has asymmetric characteristics by beam splitter, it shows the injection locking properties even without any optical isolator. In this structure, outputs of LD were coherently added and exhibited stable characteristics, e.g., broad locking range, insensitivity to unwanted weak outer feedback. This strongly coupled structure is expected to be a new candidate for a light source of the research into the secret code technology in optical communication fields, spectroscopy, and so on.
