Abstract

External optical feedback can significantly alter the operating characteristics and output radiation properties of semiconductor diode lasers. A semiconductor injection laser operating in an external cavity is attractive for numerous applications requiring greater spectral purity, frequency stability, and smaller beam divergence than can be obtained from a solitary laser diode. A linear array of diode lasers was fabricated from a bar of five, 6-µm wide, 250-µm center spacing, single-stripe emitters. The external cavity was formed by a 0.75 pitch graded-index rod lens positioned ~1 µm from the front facet of the bar array. The rod lens transfers an image of the radiating bar facet directly from the back to the front of the rod. The external cavity is therefore formed by optical feedback from the front surface of the rod with the rod lens also performing the function of the intracavity lens element. This dual function of the graded-index rod lens enables the array laser to assume a very compact form in comparison to other external cavity configurations. It was observed that the wavelength of each diode element could be controlled through proper alignment of the external cavity. Once the diode elements were operating at the same wavelength, the array could be tuned over a 15-nm range (798-813 nm). A mutual coherence interferometer was constructed to determine qualitatively the existence of a phase locked condition. A very clear interference fringe pattern was produced. Frequency locked, mutually coherent, operation of five diode elements was achieved, with external wavelength control demonstrated.

© 1985 Optical Society of America