Most phase locked semiconductor laser arrays suffer from undesirable twin lobed farfield patterns, making them unsuitable for many applications. In this thesis we make a detailed theoretical and experimental study of this problem, and solve it by tailoring the spatial gain profile across the array. We demonstrate a tailored gain chirped array which emits 450mW into a single beam 3½° wide.
Stripe geometry lasers for use in phased arrays are examined in Chapter 2, as are design considerations for evanescently coupled phased arrays. A powerful numerical method for analyzing a nearly arbitrary one-dimensional dielectric waveguide with gain and/or loss is described.
Chapter 3 analyzes in detail the simplest array of two adjacent waveguides, both real index and gain guided and both weakly and strongly coupled. Chapter 4 discusses why a uniform array has a twin lobed farfield pattern, and introduces the concept of a nonuniform real index guided chirped array of lasers with widths which increase monotonically across the array. Real index guided chirped arrays can, in principle, be made to lase with a single lobed farfield pattern. Since such arrays are difficult to fabricate, and will be at least partially gain guided, we concentrate on gain guided structures. The combination of gain tailoring and a high interchannel gain in a proton implanted chirped array enables us to achieve our goal of fabricating a high power array with the single lobed farfield pattern described above.
Such arrays are actually tailored gain broad area lasers. Chapter 5 demonstrates another method for gain tailoring, the "halftone" process, which can create nearly arbitrary two-dimensional spatial gain profiles in an optoelectronic device, thereby offering a new degree of freedom to the designer of semiconductor lasers. Single lobed nearly diffraction limited beams from tailored gain broad area lasers 50µm wide are obtained.
Asymmetric tailored gain waveguides have several unusual properties. The technique of Path Analysis for analyzing these complex waveguides is introduced. Fundamental Fourier Transform relationships relating device structure to farfield patterns yield additional insights. Finally, we close with a measurement of the antiguiding parameter and briefly examine some design criteria for practical tailored gain broad area lasers.
